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TYPES  OF  SINGLE  OPERATOR 
ARC  WELDING  GENERATORS 


BY 

MYRON  SCOTT  HANCOCK 

B.S.  University  of  Illinois,  1917 


THESIS 

SUBMITTED  IN  PARTIAL  FULFILLMENT  OF  THE  REQUIREMENTS 
FOR  THE  DEGREE  OF  ELECTRICAL  ENGINEER 
IN  THE  GRADUATE  SCHOOL  OF  THE 
UNIVERSITY  OF  ILLINOIS,  1922 


URBANA,  ILLINOIS 


— ....  • • 


I <0 


Digitized  by  the  Internet  Archive 
in  2016 


https://archive.org/details/typesofsingleopeOOhanc 


CONTENTS 


PART  I. 

ARC  WELDING  PRACTICE  AFFECTING  THE  GENERATOR 
Section  Pa^e 

1.  Definition 1 

2.  Methods  of  Arc  Welding 1 

3.  Arc  Voltage  and  Current  Characteristic  Re- 

quirements  4 

4.  Classes  of  Arc  Welding  Machines 5 

PART  II. 

REQUIREMENTS  FOR  A SINGLE  OPERATOR  GENERATOR 

5.  Volt- Ampere  Curve 8 

6.  Speed  of  Action 11 

? . Indue  t anc  e 12 

8.  Efficiency 12 

9.  Commutation 13 

1C.  Simplicity 13 

11.  Size 13 

12.  Portability 13 

13.  Cost 13 

PART  III. 

RELATION  BETWEEN  THE  GENERATOR  CHARACTERISTICS 
DESIRED  AND  THE  GENERATOR  DESIGN. 

14.  Volt- Ampere  Curve 14 

15.  Speed  of  Action 16 


- < & n waM 


Section 

IS. 

Inductance 

Pa.?e 

23 

17. 

Commutation ; 

25 

IS. 

Size 

26 

19. 

PART  IV. 

TYPES  OF  SINGLE  OPERATOR  GENERATORS 
Separately  Excited  Generators 

28 

20. 

Shunt  Wound  Seif  Excited  Generators 

31 

31. 

Self  Excited  Addative  Compound  Wound  Gener- 

ators 

35 

22. 

Separately  Excited  Differential  Compound 

Wound 

Generators 

41 

r%  «-2 

& O • 

Separately  Excited  Self  Excited  Shunt  Wound 

Generators 

47 

24. 

Separately  Excited  Self  Excited  Differential 

Compound  Wound  Generators 

50 

25 . 

Interconnected  Generators 

55 

<“}  r* 
<30 . 

Self  Excited  Differential  Compound  Wound 

Third 

Brush  Generator 

63 

27. 

Self  Excited  Differential  Compound  Wound 

Split 

Pole  Generator 

68 

28. 

PART  V. 

CONCLUSION 

Present  Stage  of  Development 

73 

29. 

Probable  Future  Development 

75 

■ 


LIST  OF  FIGURES 


F i:;;urs 


Page 


1.  Types  of  welding  generator  curves S 

2.  Welding  generator  curves  desired 15 

3.  Generator  regulation  curves 16 

4.  Separately  excited  high  resistance  gener- 

ator curves. 30 

5.  Separately  excited  adjustable  external  re- 


sistance generator  curves 32 

6.  Self  excited  addative  compound  wound  ad- 

justable external  resistance  gener- 
ator diagram 3G 

7.  Self  excited  addative  compound  wound  gen- 

erator curves 3S 

8.  Self'  excited  addative  compound  wound  gen- 

erator curves 40 

9.  Self  excited  addative  compound  wound  gen- 

erator diagram.  42 

10.  Separately  excited  differential  compound 

wound  generator  diagram 42 

11.  Curves  of  a separately  excited  differen- 


tial compound  wound  generator  with  ad- 


12. 


justable  series 

Curves  of  a separately  excited  differ  an- 


44 


tial  compound  wound  generator,  with 
separate  field  adjustable 


45 


Figure 


Pag:  s 


13.  Curves  of  a separately  excited  self  excited 

shunt  wound  generator  with  separate  field 
adjustable 48 

14.  Curves  of  a separately  excited  seif  excited 

shunt  wound  generator  with  both  fields 
adjustable 49 

15.  Diagram  of  a separately  excited  shunt  wound 

generator  with  separate  field  adjustable...  51 

16.  Diagram  of  a separately  excited  seif  excited  dif- 

ferential compound  wound  generator  with  all 
fields  adjustable 51 

17.  Curves  of  a separately  excited  self  excited  dif- 

ferential compound  wound  generator  with 
separate  field  adjustable 53 

18.  Curves  of  a separately  excited  self  excited  dif- 

ferential compound  wound  generator  with 
self  excited  shunt,  and  series  field  ad- 
justable  54 

19.  Diagram  of  a separately  excited  differential 

compound  wound  interconnected  generator 

with  both  fields  adjustable 56 

20.  Diagram  of  a separately  excited  self  excited 

shunt  'wound  interconnected  generator  with 

separate  field  adjustable 56 

31.  Diagram  of  a separately  excited  self  excited 

differential  compound  wound  interconnected 
generator  with  all  fields  adjustable 57 


Figure  Page 

22.  Diagram  of  a special  type  of  separately  excited 

self  excited  differential  compound  wound 

interconnected  generator  with  separate 

field  adjustable 57 

23.  Diagram  of  a special  separately  excited  self  ex- 

cited differential  compound  wound  intercon- 
nected generator  with  both  shunt  fields  ad- 
justable  61 

24.  Diagram  of  a self  excited  differential  compound 

wound  third  brush  generator  with  shunt  field 
adjustable 61 

25.  Generator  field  forms 65 

26.  Diagram  of  a self  excited  differential  compound 

wound  split  pole  generator  with  both  fields 
adjustable 68 

27.  Curves  of  an  unsaturated  split  pole  generator 70 

28.  Curves  of  a saturated  split  pole  generator 71 

29.  Single  operator  arc  welding  generator  comparison...  77 


TYPES  OF  SINGLE  OPERATOR  ARC  WELDING  GENERATORS 


PART  I. 

ARC  WELDING  PRACTICE  AFFECTING  THE  GENERATOR 

1.  Definition.  - The  process  of  arc  welding  has  been  de- 
fined as  the  utilization  of  the  intense  concentrated  heat  produced 
by  the  electric  arc  for  melting  and  fusing  the  metals  to  be  weld- 
ed. The  metals  instead  of  being  heated  and  forced  together,  as 

in  forge  welding,  are  melted  at  their  point  of  contact  and  the  two 
metals  fuse  without  pressure  at  this  point.  It  is  usually  neces- 
sary to  feed  extra  metal  into  the  arc,  which  melts  and  fuses  with 
the  other  molten  metal,  thus  helping  to  build  up  the  joint  between 
the  two  pieces.  Arc  welding  is  used  not  only  to  join  two  or  more 
pieces,  but  also  to  build  up  a single  piece.  In  this  case,  metal 
is  fed  into  the  arc,  melts,  and  fuses  with  the  part  of  the  piece 
to  be  built  up  that  is  melted  by  the  arc.  In  its  processes  and  re- 
sults arc  welding  is  very  much  line  casting. 

2.  Methods  of  Arc  Welding.  - Several  different  methods 
have  been  devised  for  the  use  of  the  heat  of  the  electric  arc  for 
welding.  The  method  used  in  a majority  of  cases  in  this  country 
is  the  metallic  electrode  process.  In  this  process  the  positive 
side  of  the  welding  circuit  is  connected  to  the  worx  to  be  welded. 
The  negative  side  is  connected  to  a metal  rod  or  pencil.  This  me- 
tallic rod,  forming  the  negative  electrode,  is  brought  into  con- 
tact with  the  work  and  quickly  withdrawn  a short  distance.  As  the 
electrode  is  withdrawn  an  arc  is  formed  between  the  work  and  the 
electrode,  which  melts  a spot  on  the  work  and  also  melts  the  end 


of  the  metal  red.  The  molten  material  from  the  rod  is  automatical- 
ly deposited  in  the  hottest  portion  of  the  weld  surface.  The  me- 
tallic electrode  may  be  "bare,  or  it  may  be  coated  with  some  sub- 
stance which  tends  to  prevent  oxidation  of  the  molten  metal  pass- 
ing through  the  arc.  Beth  types  of  electrodes  are  in  popular  use. 
When  the  bare  electrode  is  used,  this  process  is  sometimes  called 
the  Siaviano.ff  process*  and  when  the  coated  wire  is  used*  it  may 
be  called  the  St rohmenger- Slaughter  process. 

For  metallic  electrode  welding  with  bare  electrode  ap- 
proximately 20  volts  are  required  at  the  arc.  This  voltage  is  ap- 
proximately constant  for  any  arc  current  or  any  permissible  vari- 
ation of  arc  length.  If  coated  electrodes  are  used*  the  voltage 
required  is  higher*  usually  about  35  volts.  This  voltage,  however* 
depends  on  the  coating  used  on  the  electrode.  The  amperage  used 
depends  primarily  on  the  thickness  of  the  work.  Sufficient  current 
must  be  used  to  melt  a portion  of  the  surface  of  the  material  to  be 
welded.  Due  to  the  high  heat  storage  capacity  and  to  the  better 
heat  conduction  from  a point  in  heavy  worx*  it  is  found  that  larger 
currents  must  be  used  for  heavy  work  than  for  light.  For  1/8" 
plate* about  75  amperes  should  be  used.  For  1"  plate*  about  200  am- 
peres should  be  used.  In  metallic  electrode  welding,  the  arc 
should  be  kept  as  short  as  possible.  If  the  arc  is  long*  natural 
air  drafts  disturb  the  stationary  arc  and  tend  to  move  it  around 
an  extensive  surface  of  the  worx.  The  heat  is  net  concentrated 
and  the  surface  cf  the  work  is  not  properly  melted.  Besides,  with 
a long  arc,  the  molten  metal  in  the  arc  is  very  likely  to  become 
oxidized*  and  then  when  deposited  in  the  weld,  it  will  weaken  it. 


. 


. 


■ 


3 . 


The  arc  gap  is  usually  held  at  about  1/S"  length. 

Although  the  great  majority  of  all  electric  arc  welding  is 
done  by  the  metallic  electrode  process  the  carbon  electrode  process 
is  also  frequently  used.  In  this  process,  the  metallic  electrode 
is  replaced  by  a carbon  electrode.  After  the  arc  is  formed,  a rod 
of  metal  is  fed  into  the  arc  from  the  side  by  the  operator.  The 
heat  of  the  arc  melts  this  metal  and  it  fuses  with  the  molten  por- 
tion of  the  word.  Inasmuch  as  the  carbon  tends  to  be  carried  into 
the  weld,  the  carbon  electrode  process  produces  a weak  joint. 
Therefore,  this  process  is  not  used  where  great  strength  is  re- 
quired. It  can  neither  be  used  for  welding  on  a vertical  surface 
nor  where  the  work  is  overhead.  This  process  is  sometimes  called 
the  Bernados  process. 

For  carbon  electrode  welding,  a voltage  of  about  35  volts 
is  required.  As  in  metallic  electrode  welding,  the  thicker  plates 
require  more  current  than  the  lighter  ones.  In  carbon  arc  welding, 
usually  about  300  to  450  amperes  are  used,  although  for  light  work, 
only  200  amperes  may  be  used,  and  for  very  heavy  work,  about  800 
amperes  may  be  required.  Contrary  to  the  practice  in  metallic 
electrode  arc  welding,  with  a carbon  electrode  as  long  an  arc  as 
possible,  usually  about  1 to  1-1/3  inches,  is  held.  This  is  to 
permit  the  oxidation  of  the  carbon  in  the  arc  and  sc  prevent  its 
being  deposited  in  the  weld. 

No  other  electric  arc  welding  process  is  now  in  general 
use,  although  some  others  have  been  developed  and  used  slightly. 
They  are  of  experimental  value,  but  have  no  bearing  on  the  design 


4. 


of  commercial  arc  welding  generators. 

3.  Axe  Voltage  and  Current  Characteristic  Requirements.  - 
Both  alternating  current  and  direct  current  are  used  for  arc 
welding  and  both  give  good  welds.  A special  transformer,  instead 
of  a special  motor  generator  set,  is  required  when  alternating 
current  is  used.  The  alternating  current  apparatus  has  the  ad- 
vantages that  it  has  no  moving  parts,  no  commutation  trouble,  it 
is  light  enough  to  be  carried  by  hand  and  has  a low  initial  cost. 
The  disadvantages  of  alternating  current  are’  The  heat  is  equal 
at  both  electrodes  while  most  of  the  heat  is  needed  at  the  work; 
it  cannot  be  used  satisfactorily  for  carbon  electrode  welding 
for  the  carbon  would  be  carried  into  the  weld  when  the  carbon 
electrode  was  the  positive  terminal;  it  requires  a more  skilful 
operator  than  does  direct  current  due  to  the  voltage  dying  down 
to  zero  every  cycle;  the  weld  is  not  lively  to  be  so  good,  for, 
due  to  the  difficulty  of  welding,  the  arc  will  be  broken  and 
started  more  frequently,  which  weakens  the  weld.  As  a result  of 
these  factors,  direct  current  is  ordinarily  used  where  much  weld- 
ing is  to  be  done. 

It  is  well  known  that  if  a constant  voltage  is  impressed 
on  the  electrodes  of  an  electric  arc  light,  the  arc  is  unstable 
and  to  make  the  arc  stable,  it  is  necessary  to  work  some  adjust- 
ment so  that  the  voltage  across  the  arc  will  decrease  as  the  arc 
current  increases.  The  same  thing  is  true  of  the  electric  arc  in 
welding.  A.  drooping  volt-ampere  curve  is  necessary  to  obtain  a 
stable  arc.  Besides,  the  current  should  remain  fairly  constant 
when  welding,  in  order  to  get  a vood  weld.  It  is  inevitable  that 


5 


the  operator  will  not  be  able  to  hold  an  arc  of  absolutely  uni- 
form length.  If  the  voltage  impressed  on  the  electrodes  were  con- 
stant, the  current  would  vary  widely  as  the  arc  length  changed. 

A drooping  volt-ampere  curve  is  necessary  to  minimize  this  effect. 

For  ease  in  welding,  it  is  essential  that  the  arc  must  not 
break  if  the  length  of  the  gap  is  suddenly  increased.  Otherwise, 
it  would  be  almost  impossible  to  either  strike  or  maintain  an  arc. 
Another  difficulty  found  in  striking  an  arc  is  that  often  the 
metal  electrode,  when  used,  will  weld  to  the  work  before  it  can 
be  pulled  away.  If  there  is  even  a tending  toward  this,  it  is 
very  difficult  to  start  the  arc,  for  the  electrode,  when  broken 
loose,  will  probably  be  pulled  away  sc  rapidly  and  far  as  to 
break  the  arc.  Even  the  slightest  tendency  of  sticking  of  the 
electrode  makes  it  difficult  to  start  an  arc. 

4.  Classes  of  Arc  Welding  Machines.  - When  alternating 
current  is  used  for  welding,  a permanent  adjustable  resistance 
may  be  put  in  series  with  the  arc  and  voltage  taken  directly 
from  the  alternating  current  lines,  or  the  arc  voltage  may  be 
taken  directly  from  the  secondary  of  a constant  current  trans- 
former, the  primary  of  which  is  connected  directly  across  the 
line.  Either  type  of  apparatus  may  bo  used  to  reduce  the  voltage 
from  the  line  voltage  to  that  required  by  the  arc,  and  to  get  the 
necessary  drooping  volt-ampere  characteristic.  These  devices  are 
used  in  doing  alternating  current  welding  from  an  alternating  cur- 
rent line  only. 

If  a direct  current  line  is  available,  the  arc  current 


. 


. 


. 


G. 


used  is  always  direct  current,  due  to  its  better  welding  charac- 
teristics. The  same  adjustable  resistance  apparatus  used  for  al- 
ternating current  is  al30  used  for  direct  current  welding.  This 
apparatus  is  very  satisfactory  in  every  way  except  that  it  has  a 
very  poor  electrical  efficiency.  For  example,  if  operating  from 
a 125  volt  line, only  20  volts  across  the  arc  would  be  useful,  and 
the  105  volts  drop  in  resistance  would  be  a dead  loss.  The  elec- 
trical efficiency  in  this  case  would  be  16p.  If,  however,  the 
main  line  voltage  were  500  volts,  the  electrical  efficiency  would 
drop  to  4p.  One  of  the  types  of  apparatus  developed  to  overcome 
this  difficulty,  whenever  125  volts  is  the  potential  difference 
of  the  supply  lines,  is  the  single  operator  dynamo tor,  which  runs 
from  a 125  volt  line  and  is  designed  to  supply  the  arc  with  the 
proper  voltage  and  current  from  its  terminals.  Several  of  these 
are  on  the  market  and  their  electrical  efficiency  is  hi  h.  They, 
however,  come  outside  the  scope  of  this  discussion. 

Another  way  of  increasing  the  efficiency  is  to  have  a 
motor  generator  set  deliver  a lower  voltage  of  a constant  poten- 
tial and  use  this,  through  the  same  resistance  scheme  as  before, 
for  welding.  The  voltage  selected  for  these  sets  is  usually  60 
volts.  A lower  voltage  than  60  volts  .culd  require  so  little  re- 
sistance in  series  with  the  arc,  that  it  might  become  unstable, 
and  the  short  circuit  current  would  become  so  excessive  that  it 
would  tend  to  cause  the  electrode  to  weld  to  the  work  when  start- 
ing the  arc.  A higher  voltage  would  be  less  efficient.  With  this 
system,  the  overall  efficiency,  assuming  the  mo  tor- generator  set 

on 

efficiency  as  7G>t,  would  be  . ? x 60  x 100  = 23—1/ 3P>  as  compared 


7. 


to  the  16/a  for  resistance  alone  when  the  current  is  taken  direct- 
ly  from  a 125  volt  line,  and.  the  4$  when  from  a 500  volt  line. 

It  is  obvious  that  such  a motor  generator  set  could  be  designed 
to  run  from  any  voltage  direct  current  line,  or  any  voltage  alter- 
nating current  line,  and  also  that,  inasmuch  as  the  voltage  is 
not  affected  by  the  load,  several  men  could  use  the  same  genera- 
tor at  cnee  as  a source  of  power  for  welding.  Consequently,  this 
type  of  apparatus,  called  a multiple  operator  arc  welding  gener- 
ator, is  in  wide  use  where  several  men  are  doing  welding  work  at 
one  place.  However,  this  apparatus,  too,  is  outside  the  scope 
of  this  discussion. 

The  single  operator  motor  generator  set  is  designed  to 
give  a higher  electrical  efficiency  than  can  be  gotten  from  weld- 
ing by  means  of  resistance  only.  It  is  designed  to  furnish  di- 
rectly to  the  arc  the  volt-ampere  characteristics  required  for 
good  welding.  The  special  design  is  in  the  generator  only.  Trie 
motor  can  be  designed  to  run  from  any  type  of  power  supply,  whether 
alternating  current,  or  direct  current,  and  whatever  the  voltage. 
Since  the  generator  is  designed  to  furnish  the  volt-ampere  char- 
acteristic for  one  arc,  the  voltage  and  current  are  interdependent 
and  two  operators  could  not  satisfactorily  weld  from  one  gener- 
ator, because  the  actions  of  one  would  seriously  affect  the  power 
supply  of  the  other.  From  the  fact  that  these  generators  are  de- 
signed for  use  by,  and  can  only  be  satisfactorily  used  by,  one 
operator  at  a time,  these  generators  are  called  single  operator 
aru  welaing  generators,  This  class  of  generators  is  the  subject 
of  this  special  study. 


. 


. 


s. 


PART  II. 

REQUIREMENTS  FOR  A SINGLE  OPERATOR  GENERATOR 

5.  Volt- Ampere  Curve.  - Although  it  is  universally  agreed 
that  when  a single  generator  feeds  a single  arc  directly,  and  not 
through  any  resistance,  the  volts  should  drop  as  the  resistance 
of  the  arc  decreases,  a wide  difference  of  opinion  still  exists 
as  to  exactly  the  type  of  volt-ampere  curve  that  is  most  advan- 
tageous. One  opinion  frequently  presented  is  that  the  generator 
energy  output  should  be  constant  while  welding,  regardless  of  arc 
resistance.  This  would  mean  that  over  the  welding  voltage  range 
the  product  of  the  generator  volts  and  amperes  must  remain  prac- 
tically constant.  Such  a curve  is  shown  as  Curve  A in  Figure  #1. 
The  principal  advantage  claimed  for  such  a generator  is  that,  with 
such  characteristics,  if  the  arc  is  made  too  long  the  current  de- 
creases to  such  an  extent  that  the  arc  will  break,  and  that  if  too 
short  an  arc  is  held,  the  current  will  become  so  large  that  it 
will  melt  away  the  electrode  rapidly  till  the  normal  arc  length 
is  again  reached,  or,  in  other  words,  the  arc  length  tends  to  be 
self-regulating.  The  proponents  of  such  a generator  claim  that 
it  forces  the  holding  of  a short  arc. 

The  fact  that  the  arc  length  is  self-regulating  is  not  in 
itself  of  great  importance.  It  is  important,  however,  that  the 
arc  be  as  short  as  possible,  and  that  the  material  be  melted  and 
deposited  at  a uniform  rate.  Rue  to  the  fact  that  a generator 
must  be  designed  for  welding  any  of  many  different  thicknesses 
of  work,  it  is  necessary  to  provide  for  easy  and  close  adjustment 


* 


10 


of  the  welding  current  any  place  within  the  range  of  the  genera- 
tor, and  this  range  is  usually  made  from  about  73  amperes  weld- 
ing to  175  amperes  welding.  In  a constant  energy  generator,  it 
is  evident  that  with  a given  adjustment  of  the  generator,  and  with 
a given  electrode,  the  arc  will  tend  to  maintain  a certain  length. 
There  is  no  reason  for  thinking  however  that  this  is  a short  arc. 
It  is  likely  to  be  a long  arc  as  a short  one.  This  supposed  ad- 
vantage then  does  not  exist.  This  generator,  however,  has  three 
real  disadvantages.  Due  to  the  fact  that  lengthening  the  arc 
tends  in  a high  degree  to  cause  the  arc  to  break,  it  is  very  diffi 
cult  for  any  one  other  than  an  experienced  welder  to  use  such  a 
machine.  Also,  when  the  electrode  touches  the  work  in  striking 
the  arc,  the  short  circuit  current  is  so  high  that  it  tends  tc 
weld  the  electrode  to  the  work,  thus  making  the  starting  of  the 
arc  difficult.  Furthermore,  as  the  arc  length  changes,  the  cur- 


rent changes  greatly  and,  as  found  by  experiments  of  the  Research 
Department  of  the  Westinghouae  Electric  and  Manufacturing  Company, 
this  change  of  current  causes  the  electrode  to  melt  and  deposit  at 
a varying  rate,  which  is  detrimental  to  the  weld. 

The  Research  Department  of  the  Westinghouae  Electric  and 
Manufacturing  Company  found,  as  a result  of  tests  that  to  ;et  uni- 
form metal  deposition,  regardless  of  the  arc  length,  it  was  nec- 
essary to  have  an  approximately  constant  current  through  the  arc. 
On  first  thought,  it  would  seem  that  to  melt  iron  at  a uniform 
rate,  a constant  amount  of  heat  ould  have  to  be  generated  in  the 
arc,  or  a constant  energy  generator  required.  However,  as  the 


11 


arc  is  lengthened,  mere  heat  is  radiated  and  to  xeep  a constant 
amount  available  for  melting  the  iron,  approximately  a constant 
current  is  necessary.  A uniform  flow  of  welding  metal  is  then 
one  advantage  of  a generator  designed  for  a constant  current  over 
the  range  of  welding  voltages.  Consider in  ease  of  operation, 
the  constant  current  generator  is  superior  to  the  constant  energy 
generator,  as  an  increase  in  arc  length  does  net  tend  to  breax 
the  arc  when  judged  from  the  volt-ampere  curve  alone.  Further- 
more, it  does  not  have  so  great  a tendency  to  cause  the  electrode 
to  stick  on  starting  the  arc,  for,  with  the  same  welding  currents 
in  both  cases,  the  short  circuit  current  of  the  constant  energy 
generator  far  exceeds  that  of  the  constant  current  machine.  Any 
length  of  arc  can  be  used  with  a constant  current  generator,  as 
well  as  with  a constant  energy  generator.  Curve  B of  Figure  #1 
is  a typical  volt-ampere  curve  of  a constant  current  arc  welding 
generator . 

6.  Speed  of  Action.  - If,  when  welding,  the  length  of  arc 
is  suddenly  increased,  the  current  through  the  arc  will  momentarily 
die  down  and  then  build  up  again.  It  may  even  die  down  enough  to 
breax  the  arc.  This  will  happen  even  with  constant  current  gener- 
ators where  the  steady  current  was  the  same  for  the  original  and 
final  length  of  arcs,  and  all  lengths  between.  It  is  evident  that 
for  ease  of  operation,  these  transient  deviations  from  the  normal 
steady  conditions  should  be  minimized  as  much  as  possible,  both 
as  to  the  magnitude  of  the  deviation  and  the  length  of  time  it 
lasts.  This  is  essential,  for  otherwise,  regardless  of  the  shape 
of  the  volt-ampere  curve,  inasmuch  as  it  is  impossible  wo  al  ;ays 
hold  an  arc  of  uniform  length,  the  current  will  always  be  rapidly 
increasing  or  decreasing,  the  rate  of  material  deposition  will 


12  • 

not  be  uniform,  and  the  arc  wi'  1 break  frequently.  ■‘-Iso  when  a 
single  operator  generator  is  short  circuited  in  starting  an  arc, 
the  short  circuit  current  will  build  up  to  a value  greater  than 
the  steady  short  circuit  current,  and  then  will  die  down  to  that 
value.  This  deviation  from  normal  short  circuit  current  should 
also  be  minimized,  as  an  excessively  high  current  tends  to  cause 
the  electrode  to  stick  to  the  ork.  To  reduce  these  variations, 
it  is  necessary  tc  have  a fast  field,  sc  that  after  any  change 
in  arc  length,  the  main  pole  flux  will  reach  to  its  steady  value 
as  scon  as  possible. 

7.  Inductance.  - Another  condition  that  usually  will  give 
much  the  same  effect  as  a fast  field,  is  to  have  high  inductance 
in  the  armature  circuit,  this  inductance  may  come  from  the  gen- 
erator itself  or  may  be  a separate  external  inductance.  Induc- 
tance in  the  armature  circuit  reduces  the  transient  variations, 
tends  to  hold  the  current  steady,  and  so  makes  for  ease  in  well- 
ing. In  this  way,  it  may  act  as  a partial  substitute  for  a fast 
field.  when  striking,  the  arc,  however,  the  inductance  will  tend 
tc  prevent  the  current  from  exceeding  normal,  but  it  also  causes 
the  current  to  build  up  to  normal  slowly,  which  is  not  desired. 

8.  Efficiency.  - A high  electrical  efficiency  is  desira- 
ble in  a welding  generator,  but  its  importance  is  frequently  over- 
estimated. The  cheapest  generator  is  the  one  which  will  do  the 
most  welding  with  the  least  total  cost.  Assuming  an  arc  welding: 
motor  generator  set  overall  efficiency  of  55P,  a power  cost  of 

3 cents  per  kilowatt  hour  and  an  average  generator  output  of  2.2 
K.‘.r.,  the  cost  of  power  for  one  welder  ould  be  12  cents  a...  hour. 
The  welder’s  time  probably  would  cost  73  cents  an  hour.  If  by 


v 


13. 

any  change  in  design  of  the  generator  the  welder  were  enabled  to 
dc  10/=  more  ./ora  than  before,  the  saving  in  the  amount  paid  the 
welder  would  justify  a decrease  in  the  motor  generator  electrical 
efficiency  from  55%  to  3 5p.  Electrical  efficiency  is,  therefore, 
a poor  criterion  of  the  worth  of  an  arc  welding  generator. 

9.  Commutation.  - A single  operator  arc  welding  generator 
is  subject  to  high  peak  currents.  To  care  for  these  peak  currents 
ana  the  rapidly  fluctuating  load  satisfactorily,  the  commutation 
characteristics  of  the  generator  should  be  very  good. 

10.  Simplicity-  To  reduce  the  probability  of  incurring 
trouble  with  the  generator  and  to  facilitate  the  generator's  re- 
pair when  it  does  give  trouble  the  generator,  its  panel,  and  all 
connections  should  be  as  simple  as  possible. 

11.  Size.  - The  generator  and  its  auxiliary  equipment 
should  be  as  small  and  light  as  is  consistent  with  good  design. 

Such  a generator,  with  its  auxiliary  equipment  and  driving  motor, 
is  often  mounted  on  trucks  and  should  be  light  enough  to  be  handled 
with  ease,  and  should  be  small  in  dimensions  so  that  it  can  be 
moved  readily  through  crowded  shop  aisles. 

IS.  Portability.  - As  the  generators  and  auxiliary  equip- 
ment are  often  mounted  on  trucks  and  hauled  around,  the  mechanical 
design  of  the  equipment  should  be  made  with  such  service  in  view. 
All  parts  should  be  designed  for  portable  service. 

13.  Cost.  - For  commercial  reasons,  it  is  essential  that 
the  first  cost  of  the  welding  generator  and  auxiliary  equipment 
be  as  low  as  is  consistent  with  good  design. 


14. 


PART  III. 

RELATION  BETWEEN  THE  GENERATOR  CHARACTERISTICS 
DESIRED  AND  THE  GENERATOR  DESIGN. 


14.  Volt- Ampere  Curves . - In  designing  a single  operator 
welding  generator,  the  method  of  attack  is  usually  to  design  to 
obtain  the  desired  volt- ampere  curves  and  then  to  modify  this 
design  in  such  a way  as  to  obtain  the  other  characteristics 
desired.  The  important  differences  in  the  various  types  of  ma- 
chines are  those  due  to  the  different  methods  of  obtaining  the 
desired  volt- ampere  curves. 

Ordinarily,  in  the  design  of  a generator  to  get  any  desired 
volt- ampere  curve,  the  designer  may  make  use  of  seven  different 
means  of  shaping  the  curve.  He  may  use,  if  he  desires,  a sepa- 
rately excised  field,  a self-excited  shunt  field,  a series  field, 
either  addative  or  differential,  and  may  select  such  a saturation 
curve  as  he  desires. 


These  are  his  four  chief  means.  He  may  also  make  use  to  a 
much  smaller  degree  of  shifting  the  brushes  from  neutral,  making 
the  commutating  pole  winding  either  too  strong  or  too  weak,  and 
increasing  the  distorting  effect  of  the  armature  ampere  turns. 

If  the  generator  is  made  a freak,  as  fox  example  using  a four 
pole  stator  with  a two  pole  rotor,  further  possibilities  of  vari- 
ation of  the  volt-ampere  curves  are  found. 

The  ideal  volt-ampere  curves  to  be  produced  by  the  different 


adjustments  of  a single  operator  generator  are  shown  in  Figure  #2. 

In  Figure  #3  is  shown  the  typical  volt  ampere  curves  cf  a 
separately  excited  generator,  a shunt  wound  generator  self-excited. 


17. 


and  a series  wound  generator.  The  various  combinations  possible 
of  these  windings,  with  their  advantages  and  disadvantages,  will 
be  discussed  later. 

When  working  high  on  the  saturation  curve,  as  load  is  ap- 
plied, the  voltage  does  not  decrease  so  rapidly  as  when  working- 
on  the  straight  line  part  of  the  curve.  To  obtain  the  type  of 
volt- ampere  curves  desired,  then,  the  generator  should  be  sat- 
urated at  no  load,  but  unsaturated  at  welding  voltage.  The  curve 
resulting,  would  be  approximately  os  shown  in  Figure  #2. 

Shifting  brushes  slightly  from  neutral  gives  approximate- 
ly the  same  effect  as  the  addition  of  a series  field.  A shift 
in  the  direction  of  rotation  gives  the  effect  of  a differential 
series.  A shift  in  the  opposite  direction  gives  the  effect  of 
an  addative  series.  The  effect  to  be  gained  in  this  way  is  limi- 
ted, for  if  the  brushes  are  shifted  far  from  neutral,  commutation 
will  become  bad.  In  the  same  way,  if  the  commutating  ceil 
strength  is  made  more  than  is  required  for  best  commutating  con- 
ditions, the  resultant  circulating  currents  flowing  in  the  coils 
short  circuited  by  the  brush  have  much  the  same  action  on  the 
main  pole  flux  as  would  an  addative  series  on  the  main  pole  it- 
self. Ma  ing  the  commutating  coil  too  weak,  would  have  an  effect 
similar  to  using  a differential  series  coil.  Here  again,  commu- 
tation is  a limit . 

V*'hen  a generator  is  loaded,  the  armature  ampere  turns  in- 
crease the  flux  at  one  tip  of  each  pole  and  decrease  the  flux 
at  the  other  tip.  Due  to  the  shape  of  the  iron  saturation  curve, 
the  increase  of  the  flux  is  not  sc  great  as  the  decrease  and,  as 
a result,  the  total  flux  through  the  pole  is  decreased  by  the 


18 


armature  distorting  ampere  turns.  This  reduction  of  flux  is  in- 
creased by  using  a smaller  air  gap  and  decreased  by  using  a larger 
gap,  but  it  is  not  possible  to  vary  this  reduction  much  by  such 
means . 

15.  Speed  of  Action.  - The  speed  of  the  field  action  is 
secondary  in  importance  only  to  the  volt-ampere  curve  of  a single 
operator  arc  welding  generator*  Two  conditions  must  be  obtained 
if  after  a change  of  arc  length,  the  arc  voltage  and  current  are 
to  quickly  reach  their  steady  value.  The  first  condition  is  that 
the  main  pole  flux  changes  must  be  damped  as  little  as  possible. 
Damping  of  the  rate  of  flux  change  cannot  be  entirely  eliminated. 
The  most  effective  dampers  on  the  rate  of  flux  changes  are  the 


shunt  field  windings. 

With  any  definite  rate  of  change  of  flux  in  the  main  poles, 
the  volts  induced  in  all  the  shunt  coils  connected  in  series,  is 
Kn  • 


Where  K is  a constant  depending  on  the  rate  of  flux  change 
and  the  number  of  coils  in  series, 
and  N is  the  number  of  turns  in  each  of  the  shunt  coils. 
Also,  the  damping  current  caused  to  flow  by  such  voltage  is 
where  R is  the  resistance  of  the  shunt  field  circuit.  The  damp- 
ing ampere  turns  per  ceil  then  would  be  ^ . These  ampere  turns 
are  superimposed  directly  on  the  ampere  turns  from  the  steady  ex- 
citation voltage. 


it 

and 

The 


To  obtain  little  damping 
is  necessary  that  that  shunt 
that  it  have  few  turns  inter 
resistance-  of  the  coils  them 


action  from  any  shunt  coil,  then 
coil  circuit  have  a high  resistanc 
linking  with  the  magnetic  circuit. 


selves  would  be 


N 


K jA  J 


e 


’where  kq  i g 


19* 

approximately  a constant,  and  is  dependent  on  the  coil  dimensions, 
and  number  of  coils  in  series,  and  A is  the  cross  section  of  the 
conductor.  Then,  not  including  any  external  resistance  in  series 
with  the  coils,  the  damping  ampere  turns  would  become  KK^KA. 

But  NA  is  the  total  cross  section  of  copper  in  a coil,  and  there- 
fore the  damp in  action  is  proportional  to  the  cross  section  of 
the  copper  in  the  coil,  if  no  external  resistance  is  used.  On 
account  of  the  damping  action,  the  cross  sec  ion  of  all  shunt 
coils  should  be  made  as  small  as  possible.  Such  reduction,  how- 
ever, is  limited.  A definite  number  of  ampere  turns  is  required 
from  each  coil  and  the  current  density  in  the  coil  is  limited  by 
heating.  The  coil  ampere  turns  in  a given  machine  are  practically 
proportional  to  coil  copper  cross  section  times  current  density 
in  the  coil.  When  the  density  is  made  as  high  and  the  ampere  turns 
as  low  as  permissible,  the  cross  section  of  the  coil  is  a minimum, 
and  the  damping  action  of  the  coils  can  no  longer  be  reduced  with- 


out the  use  of  external  resistance. 

It  is  evident  that  if  an  external  resistance  of  an  amount 
equal  to  the  total  resistance  of  the  coils  in  series  is  placed  in 
series  with  the  coils,  and  if  the  excitation  voltage  is  doubled 
to  correspond,  the  ampere  turns  and  density  remain  unchanged,  but 
the  damping  ampere  turns  are  halved.  Let  the  ampere  turns  re- 
quired divided  by  the  current  density  be  KglJA,  where  Kg  is  a con- 
stant depending  on  units  used.  Then  the  damping  ampere  turns, 

with  no  external  resistance  used  is  ~~ ■ * . Let- 

Kg  Current  Density 

ting  R be  the  resistance  of  all  such  coils  in  series,  and  Rx  be 
the  total  external  resistance  in  series  with  these  coils,  then 


the  damping  ampere  turns,  when  using  external  resistance. 


are 


* 


. 


. 


' 


20 


K&1  x Ampere  Turns 


x R 


£3  Current  Density  R+Rx 


However,  the  addition  of  the 


external  resistance  entails  extra  losses  and  so  reduces  the  gen- 
erator efficiency. 

The  relation  found  above  holds  true  whether  the  shunt  is 
self  excited  or  separately  excited,  and  for  any  voltage  of  excita- 
tion. If  the  ampere  turns  are  taken  as  the  total  ampere  turns 
in  the  generator  due  to  that  shunt  circuit,  and  R is  taken  as  the 
total  internal  resistance,  as  coils  are  finally  connected,  then 
the  relation  also  holds  true  regardless  of  whether  all  coils  are 
in  series,  all  are  in  parallel,  or  there  is  a series  parallel  ar- 
rangement. If  two  separate  shunt  circuits  are  in  the  generator, 
as  a separately  excited  field  and  a self  excited  field,  then  the 
damping  ampere  turns  of  the  two  must  be  added  to  get  the  total 
shunt  damping  ampere  turns. 

The  series  coils  also  conform  to  the  same  relation  as 
the  shunt  coils.  However,  the  external  resistance,  all  resis- 


tance in  the  armature  circuit,  is  usually  so  high  that 


R 


R+Rx 


13 


so  small  that  the  series  damping  action  is  negligible.  If  the 
series  coils  are  shunted  Rx  may  become  small  and  then  the  series 
coil  damping  action  may  become  important. 

The  armature  coils  short  circuited  by  the  brushes  also 
interlink  with  the  main  pole  flux,  and  form  a closed  circuit,  and 
so  act  as  dampers.  Due  xo  the  fact  that  the  turns  involved  are 
few,  and  the  brush  contact  drop  is  high  compared  to  the  resistance 
of  an  armature  coil,  it  is  probable  that  the  damping  effect  of 
the  short  circuited  armature  coils  is  negligible. 

If  the  brushes  are  off  neutral  the  armature  will  have 


‘ 


' 


.. 


. 


' 


21. 


an  additional  damping  effect  similar  to  that  of  the  series  coils, 
and  negligible  for  the  same  reasons. 

The  iron  parts  of  the  magnetic  circuit  also  tend  to  damp 
out  any  changes  in  flux.  A flux  change  sets  up  eddy  currents  in 
the  iron  which  tend  to  prevent  the  change.  By  laminating  the  iron 
such  eddy  currents  are  largely  eliminated.  In  ail  modern  gener- 
ators the  armature  and  main  poles  are  laminated.  Laminating  the 
frame,  also,  should  tend  to  reduce  the  damping  action,  but,  as 
iron  gives  a very  high  resistance  patch  to  eddy  currents,  the 
damping  effect  of  the  generator  frame  is  probably  negligible.  Cer- 
tainly it  is  not  of  the  magnitude  of  the  damping  effect  of  the 
shunt  coils.  Consequently,  it  is  doubtful  if  laminating  the  frame 
is  justifiable. 

The  second  factor  affecting  the  speed  of  the  change  of 
flux  is  the  ampere  turns  on  the  generator  available,  for  causing 
an  increase  or  decrease  of  flux,  at  each  instant  during  the  time 
the  flux  is  changing  from  one  steady  value  to  another.  A gener- 
ator in  which  the  flux  change  is  dependent  on  the  change  of  a 
self  excited  shunt  field  excitation  will  be  extremely  slow  in  ac- 
tion, for  an  increase  in  generator  voltage  results  in  turn  in  an 
increase  of  excitation,  an  increase  in  flux,  and  an  increase  in 
voltage,  and  so  on  around  the  circle.  The  increase  of  excita- 
tion comes  slowly,  and  by  small  increments,  and  therefore  the  flux 
builds  up  slowly.  This  is  such  a well  known  condition  that  no  ex- 
planation is  necessary. 

If  the  flux  change  is  due  to  the  change  of  current  through 
a differential  series  field  the  action  should  be  extremely  rapid. 
Assume  that  the  arc  is  suddenly  lengthened  while  welding,  then 


. 


- 


. 

. 


> 

. 


£>4>  • 

the  current  will  suddenly  drop  to  an  amount  less  than  the  final 
steady  welding  current  at  exactly  the  same  time  the  net  field 
ampere  turns  increase  till,  at  the  instant  the  final  steady  weld- 
ing current  is  reached,  the  field  would  have  reached  its  final 
ampere  turns,  except  for  the  damping  effect  of  the  separately  ex- 
cited field.  Due  to  this  damping  effect,  the  separately  excited 
field  ampere  turns  will  be  less  than  the  final  steady  value, 
and  so  the  current  continues  to  decrease  further.  However,  the 
rate  of  change  slows  up,  the  damping  effect  decreases,  and,  as  the 
separately  excited  field  overcomes  the  damping  effect,  the  current 
increases  and  the  differential  series  field  approaches  its  final 
value.  On  the  other  hand,  if  the  are  is  suddenly  short  circuited 
the  differential  series  ampere  turns  will  increase  exactly  as 
does  the  armature  current,  and  when  the  armature  current  overshoots 
the  final  value,  so  will  the  differential  series.  It  can  be  seen 
that  the  differential  series  field  is  ideal  for  getting  quick  flux 
changes  for  it  acts  exactly  at  the  instant  the  current  changes, 
acts  always  in  a way  to  bring  the  current  to  its  final  steady 
value,  and  the  magnitude  of  the  magnetomotive  force  due  to  the 
series  tending  to  bring  the  flux  to  normal,  is  always  proportional 
to  the  amount  the  current  is  different  from  normal. 

An  addative  series  field  acts  in  every  way  the  exact  op- 
posite of  a differential  series.  When  the  current  decreases,  it 
causes  the  field  flux  to  reduce,  but  it  should  be  doing*  the  re- 
verse. When  the  current  increases,  it  causes  the  field  flux  to 
increase,  but  it  should  decrease,  and  must  before  a stable  welding 
condition  is  reached. 


23. 

Shifting  brushes  from  neutral,  and  making  the  commutat- 
ing ceil  toe  strong,  or  weak,  has  the  same  effect  as  the  use  of 
a series  field,  as  explained  previously. 

Summing  up,  in  order  to  obtain  rapid  flux  changes,  the 
principal  pc int s to  be  observed  are: 

(1)  Use  as  low  a maximum  total  shunt  field  ampere 

turns  as  possible. 

(2)  Use  as  high  a current  density  in  the  shunt 

field  as  possible. 

(o)  Use  as  much  external  resistance  in  series  with 
the  shunt  fields  as  permissible. 

(4)  Use  as  strong  a differential  series  field  as 

permissible. 

(5)  Avoid  using  any  self  excited  shunt  fields  in 

which  the  excitation  is  caused  to  change 
by  a change  in  arc  length. 

(s)  Uo  not  use  an  addative  series  field. 

The  excitation  of  a separately  excited  shunt  field  does 
not  change  with  the  change  of  arc  length,  so  the  only  effect  such 
a field  has  on  the  speed  of  flux  change  is  its  damping  effect. 

16.  Inductance.-  '.'■’hen  an  armature  is  loaded  a strong 
cross  magnetic  flux  is  set  up.  This  flux  enters  one  tip  of  each 
pole,  traverses  the  pole  to  the  other  pole  tip,  crosses  the  air 
P to  the  armature  and  returns  through  the  armature  core.  This 
cross  magnetic  flux  is  approximately  proportional  to  the  armature 
current  and  interlinks  v.ith  many  of  the  armature  conductors.  It 
is  evident  that  this  flux  makes  the  armature  inductive  and  that 
as  the  main  pole  airgap  becomes  smaller  the  cross  flux,  and,  in 


. 


♦ 


IhH 

1 it 8 H ■ 


24. 


turn,  the  armature  inductance  increases.  As  the  armature  turns 
increase  both  the  flux  and  turns  interlinked  increase,  and  so  the 
inductance  is  approximately  proportional  to  the  square  of  the 
armature  turns.  This  presupposes  that  the  pole  tips  are  not  suf- 
ficiently saturated  as  to  materially  reduce  the  cross  flux.  The 
poles  should  be  designed  so  that,  with  the  maximum  possible  cross 
flux,  there  would  be  no  pole  tip  saturation. 

A second,  although  much  less  important,  source  of  in- 
ductance is  the  series  field.  In  general,  as  the  armature  cur- 
rent increases  the  field  flux  decreases,  and  vice  versa.  It  is 
desired  to  keep  the  armature  current  constant.  The  inductive 
voltage  set  up  in  a coil  by  the  change  of  flux  interlinking  with 
that  coil  is  always  such  as  to  tend  to  cause  such  a current  to 
flow  in  the  coil  as  would  tend  to  prevent  a change  of  flux.  Assum- 
ing these  three  things,  it  is  evident  that  when  a differential 
series  field  is  used: 

(1)  An  increase  of  current  causes  a decrease  of  flux. 

(2)  A decreasing  flux  sets  up  an  induced  voltage  in 

the  series  coil. 

(3)  This  induced  voltage  tends  to  cause  current  to 

flow  in  such  a way  that  it  will  magnetize 

the  field,  opposing  the  load  current. 

(4)  Therefore  this  induced  voltage  tends  to  keep  the 

armature  amperes  from  increasing. 

This  coil  differs  in  action  from  an  ordinary  inductance, 
in  that  the  flux  is  not  proportional  to  the  coil  current.  Other- 
wise, the  effect  is  the  same.  In  an  addative  series  field  used  on 
a generator  in  which  the  flux  decreases  with  an  increase  of  load 


' 


25. 

the  induced  voltage  tends  to  increase  the  series  co.il  current,  and 
so  the  effect  is  the  reverse  of  inductive. 

A third  source  of  inductance  is  the  slot  flux,  flux  leaks 
across  the  top  and  through  the  middle  of  a sloipand  interlinks  with 
the  armature  coils.  This  inter linkage  adds  to  the  total  inductance 
of  the  armature.  It  can  be  increased  by  deepening  the  slot  and 
making  it  narrower,  but  such  change  tends  to  cause  a higher  react- 
ance voltage  and  make  the  commutation  bad,  so  in  actual  design 
no  attempt  is  made  to  increase  armature  inductance  in  this  way. 

A fourth  source  of  inductance  is  the  commutating  pole 
flux.  The  commutating  pole  flux  interlinks  with  the  commutating 
pole  turns  and  with  the  armature  conductors.  It  has  an  inductive 
effect  on  the  commutating  pole  coils  out  the  effect  is  reversed 
on  the  armature  coils.  As  the  total  commutating  pole  coil  turns 
always  exceed  the  effective  armature  turns,  the  net  result  is 
that  there  is  an  inductive  effect  from  the  commutating  pole  flux, 
and  this  effect  can  be  made  a maximum  by  using  the  maximum  possi- 
ble length  of  commutating  pole  gaps,  with  the  resultant  large  num- 
ber of  commutating  pole  turns. 

17.  Commutation.  - To  insure  good  commutation,  the  arc 
welding  generator  should  be  a commutating  pole  machine.  It  is 
advisable  for  such  service  to  use  as  many  commutating  poles  as 
main  poles.  Care  should  be  taken  to  get  the  proper  shape  of  com- 
mutating pole  face  and  to  obtain  the  right  commutating  coil 
strength  for  the  gap  used.  The  brushes  should  be  set  approximate- 
ly on  the  mechanical  neutral.  The  current  density  in  the  brush 
contact  surface  should  not  be  excessive.  The  commutating  zone 
should  not  be  too  large  a part  of  the  neutral  zone. 


36. 


In  all  the  following  diagrams  for  simplicity  the  commu- 
tating pole  coils  are  not  shown,  although  they  should  be  used  in  ' 

all  cases. 

18.  Size.  - All  of  the  other  listed  requirements  of  a 
good  welding  generator  (except  simplicity,  of  which  nothing  fur- 
ther will  be  said)  are  closely  related  to  the  size. 

The  size  of  a generator  depends  very  directly  on  ixs  ef- 
ficiency. The  losses  that  can  be  dissipated  by  an  armature,  with- 
out exceeding  a given  temperature  rise,  are  approximately  proi^cr- 
tional  to  the  diameter  of  the  armature  core  multiplied  by  its 
length.  Therefore  the  less  efficient  the  generator  is  the  greater 
are  its  losses,  and,  consequently,  the  larger  must  be  the  machine. 

Assuming  the  same  general  construction,  the  larger  the 
generator  the  greater  will  be  its  cost. 

Although  both  size  and  method  of  construction  effect  the 
portability  of  a generator,  it  is  evident  that  a generator  must  be 
small  if  it  is  to  be  portable. 

Since  size  is  important,  it  is  desirable  that  the  mini- 
mum possible  size  be  determined.  Every  generator  is  expected  to 
deliver  a definite  voltage  at  a certain  speed.  It  also  will  have 
a definite  current  which  it  must  deliver  for  a certain  length  of 
time,  without  exceeding  a fixed  temperature  rise.  The  normal  sin- 
gle operator  arc  welding  generator  is  usually  expected  to  deliver 
about  55  volts  at  no  load  continuously,  and  to  deliver  about  175 
amperes  at  20  volts  for  one  hour  without  exceeding  a temperature 
rise  of  50°  C.  Taking  such  a generator  as  an  example,  it  is  seen 
that  the  generator  must  have  a flux  capacity,  when  fully  saturated. 


. 


* 


. 


. 


. 


, 


27 


sufficient  turns  on  the  armature,  to  give  55  volts  at  the  speed 
the  generator  is  to  be  driven.  Also,  the  armature  conductors  must 
be  of  such  a size  that  the  armature  will  curry  175  amperes  with- 
out exceeding  the  temperature  guarantees.  The  size  of  this  ma- 
chine would  be  practically  the  same  as  a standard  shunt  motor 
having  an  input  of  175  amperes  and  a terminal  voltage  of  55  volts 
and  no  load  speed  the  same  as  the  speed  of  the  generator.  Assum- 
ing a motor  efficiency  of  85/-  and  that  the  full  load  speed  cf  the 

motor  would  be  92 p of  the  no  lead  speed,  then  such  a generator 

• TVS  x S S v 

would  correspond  to  a motor  with  an  output  of  — w-  horee- 

power,  or  about  11  horsepower,  at  92p  of  the  generator  speed.  The 
size  motor  to  do  this  would  also  deliver  12  horsepower  at  the  gen- 
erator rated  speed,  and  that  size  machine  is  the  smallest  that 
could  be  used  to  get  the  specified  performance. 

If  other  voltages  and  amperes  are  required,  the  smallest 
standard  size  machine  that  can  be  used  may  be  determined  in  the 
same  way.  The  rating  worked  out,  55  volts  no  load,  175  amperes 
welding,  is  a normal  single  operator  generator  raxing  but  other 
ratings  are  sometimes  used. 

It  should  not  be  inferred  that  all  generators  are  made 
the  minimum  size  possible  for  their  rating.  Very  few  are  so  small. 
Most  welding  generators  have  had  their  size  increased  over  the  min- 
imum, due  either  to  the  type  field  windings  used,  the  attempt  to 
get  a high  inductance,  or  to  use  a standard  armature  with  a long 
enough  commutator  to  carry  the  rated  current.  However,  the  smal- 
lest possible  size  is  a standard  against  which  the  actual  sizes 
may  \ e checked,  and  for  comparison  will  be  referred  to  hereafter 
as  the  standard  size  generator. 


PART  IV. 

TYPES  OF  SINGLE- OPERATOR  GENERATORS 


28. 


19.  Separately  Excited  Generators.  - One  of  the  simplest 
of  the  types  of  generators  that  could  be  used  for  arc  welding  is 
a separately  excited  generator.  Such  a generator  might  be  de- 
signed for  use  with  an  external  resistance  in  series  with  the  arc, 
or  for  use  without  such  a resistance. 

If  the  generator  is  designed  for  use  without  external 
resistance,  the  resistance  of  the  generator  itself  must  be  suffi- 
cient to  cause  the  drop  from  no  load  voltage  to  welding  voltage. 
With  a definite  no  lead  voltage,  as  the  resistance  is  fixed,  a 
definite  welding  current  would  result.  In  this,  and  all  other 
types  of  welders,  it  will  be  assumed  that  the  maximum  no  load  volts 
required  are  55  and  the  maximum  welding  current  at  20  volts  is  175 
amperes.  Then  such  a generator  must  have  enough  resistance  in 
the  armature,  commutat ing  pole  coils,  and  brushes,  to  cause  a 
drop  of  approximately  35  volts  with  175  amperes  flowing.  This 
would  result  in  a loss  of  approximately  6125  watts  in  the  genera- 
tor itself.  The  minimum  size  generator  was  taxen  as  one  which 


should  have  Sop  efficiency  on  a basis  of  55  volts  output,  or  the 

losses  there  would  be  f-N.  ^-,.2,-9.  Watts  or  1444  watts.  Since 

1UU 

the  losses  that  can  be  dissipated  with  a given  rise  by  generators 
are  approximately  proport ional  to  the  diameter  times  the  length 
of  the  armature  core,  then  a separately  excited  generator  used 
without  external  resistance  would  require  an  armature  having  a 
diameter  and  length  each  about  twice  that  of  the  standard  generator. 
It  would  then  actually  be  eight  times  the  size. 


I 


. 


Inasmuch  as  for  welding  practice  the  generator  must  never 
have  a no  load  voltage  below  35  volts,  as  this  is  necess  ry  to 
strike  the  arc,  it. can  be  seen  that  the  minimum  possible  .elding 
current  would  be  110  amperes.  It  is  usually  desired  to  be  able 


to  go  down  to  ?5  amperes. 

The  volt-ampere  curve  that  would  be  gotten  is  shown  in 
Figure  #4,  and  is  entirely  satisfactory.  It  is  a straight  line 
curve,  .hi oh  is  a mean  between  the  constant  energy  and  constant 
current  curves . Ouch  a machine  would  have  little  flux  change  so 
it  i.  not  affected  by  the  speed  of  flux  chain's.  Since  it  has  no 
flux  lag  it  needs  little  inductance.  That  inductance  is  required 
is  simply  to  prevent  the  current  building  up  to  such  a value, 
when  the  electrode  is  touched  to  the  work,  in  starting  the  arc, 
as  will  weld  the  electrode  to  the  >ork.  As  can  be  seen  from 
Figure  #4,  the  short  circuit  current,  far  exceeds  the  welding  cur- 
rent. Such  a generator  is  very  inefficient  for  under  maximum 

current  conditions,  the  efficiency  is  approximately  — ■■  A.— or 

oo 

36/?.  hue  to  its  size  it  is  costly  and  not  very  portable. 

Such  a generator,  however,  is  simple  and  requires  little 
auxiliary  apparatus.  Aside  from  voltmeter  an  ammeter,  and  a 


knife  switch,  which  are  required  for  all  generators,  all  that  is 
required  is  a source  of  separate  excitation,  a field  rheostat, 
and  possibly  a reactor.  This  type  of  generator  h„s  never  been 
made  commercially  for  arc  welding. 

The  separately  excited  generator  designed  for  use  with 
external  resistance  is  designed  tc  be  as  small  as  possible  and 
to  have  an  external  resistance  use  up  the  difference  between  the 

generator  .volts  and  the  welding  voltage.  The  generator  .ill  >e 


31. 


as  small  as  the  standard  generator.  It  is  very  simple,  and  there- 
fore cheap  and  portable.  The  generator  efficiency  is  high  but  the 
total  efficiency  when  resistance  losses  are  included  is  again 
about  36/0. 

The  field  flux  does  not  change  appreciably,  and  so  speed 
of  flux  change  need  not  be  considered.  Little  inductance  is  re- 
quired, and  that  only  because  of  the  high  short  circuit  current. 
Since  practically  all  of  the  voltage  decrease  is  due  to  resistance 
drop  it  can  be  readily  seen  that,  neglecting  the  inductive  effects, 
the  voltage  increases  almost  exactly  as  the  current  decreases. 

The  volt-ampere  curves  of  such  a generator  are  shown 
in  Figure  #5.  These  are  desirable  curves. 

Such  a generator  requires  for  auxiliary  apparatus,  aside 
from  a voltmeter  and  ammeter,  a field  rheostat,  an  adjustable 
cast  grid  resistance,  with  knife  switches,  and  possibly  a reactor. 
The  big  steps  of  welding  current  would  be  obtained  by  varying  the 
resistance  in  the  armature  circuit  by  means  of  the  knife  switches, 
and  the  finer  adjustments  would  be  gotten  by  means  of  the  field 
rheostat.  Also  a separate  source  of  excitation  must  be  available. 
This  generator  has  never  been  built  commercially  for  welding  ser- 
vice . 

20.  Shunt  Wound  Self  Excited  Generators.  - If  a shunt 
wound  self  excited  generator  is  short  circuited  the  current  dies 
down  to  a low  value,  the  terminal  voltage  becomes  zero  and  there 
is  no  current  through  the  shunt  field.  Then  if  the  short  circuit 
is  quickly  removed,  as  in  starting  an  arc,  there  is  not  sufficient 
current  flowing  to  start  an  arc,  and  there  is  but  little  in- 
duced voltage  to  start  it.  As  a rsult  the  arc  break®.  To  overcome 


33 


this  it  is  necessary  to  put  a resistance  in  series  with  the  arc, 
and  also  in  most  cases,  to  put  a reactor  in  series  with  it.  The 
greater  is  the  resistance  the  larger  will  be  the  short  circuit 
current  with  the  same  field  setting,  and  the  smaller  need  be  the 
inductance  of  the  reactor.  Two  general  types  of  self  excited 
shunt  .cund  generators . with  external  resistance  can  be  used  for 
arc  welding. 

One  type  of  self  excited  shunt  wound  generator  has 
characteristics  similar  to  these  of  the  separately  excited  gener- 
ator using  external  resistance.  It  has  the  shunt  field  adjusted 
to  give  approximately  55  volts  no  load,  and  the  big  step  adjust- 
ments of  the  welding  current  are  made  by  varying  the  resistance 
in  series  with  the.  arc,  and  the  finer  adjustments  made  by  use  of 
the  field  rheostat.  Part  of  the  drop  from  55  volts  to  20  volts 
comes  from  the  natural  voltage  drop  of  a shunt  generator,  but  the 
major  part  comes  from  the  drop  in  the  external  resistance.  Since 
part  of  the  drop  of  the  voltage  is  due  to  a decrease  in  flux  the 
efficiency  would  be  somewhat  above  38>  and  the  response  of  the 
voltage  to  change  of  current  would  be  a little  sLower  than  in  the 
case  of  the  sepal’ at  el  y excited  generator. 

Line  the  separately  excited  generator  with  external  re- 
sistance, this  generator  is  the  minimum  possible  in  size  and  cost, 
is  portable  and  simple,  hue  to  the  lag  of  the  shunt  field,  it 
will  need  more  inductance  than  the  separately  excited  generator. 


It  will  require  exactly  the  same  auxiliary  apparatus  except  that 
no  separate  source  of  excitation  is  required.  Its  volt-ampere 
curves  are  practically  the  same  as  those  of  the  - separately  excit 
generator  with  external  resistance,  shown  in  Figure  if 5.  Such  a 


34 


generator  has  never  been  manufactured  commercially  fox  arc  welding 
use. 

The  second  type  of  the  self  excited  shunt  wound  genera- 
tors is  one  with  which  enough  external  resistance  and  reactance 
is  used  to  enable  an  arc  tc  be  struck.  The  external  resistance 
is  constant  in  value  and  the  drop  in  voltage  is  principally  the 
result  of  the  inherent  drooping  characteristic  of  the  shunt  gen- 
erator. The  current  adjustment  is  made  by  means  of  the  shunt 
field  rheostat.  The  no  lead  voltage  varies  widely  but  would 
probably  not  be  below  35,  when  adjusted  for  75  amperes  welding 
current. 

Such  a generator,  shunt  wound,  not  very  highly  satur- 
ated and  with  a wide  range  of  flux  values,  would  necessarily  have 
such  a sluggish  field  that  it  could  not  compete  with  other  types. 
Due  to  this  sluggishness,  it  would  require  a highly  inductive  re- 
actance in  series  with  the  arc.  It  would,  however,  be  compara- 
tively efficient,  for  much  of  the  voltage  change  is  the  result 
of  the  change  of  flux,  instead  of  resistance  drop.  It  is  more 
simple  than  the  first  self  excited  generator  described,  for  it 
uses  a constant  instead  of  an  adjustable  resistance.  Otherwise 
the  same  auxiliary  apparatus  is  used. 

Ouch  a generator  would  be  slightly  larger,  probably 
about  2 Op,  than  the  standard  generator,  for  it  could  not  be 
worked  at  as  high  a degree  of  saturation  as  iron  can  be  worked* 

If  the  saturation  be  too  high,  the  voltage  at  the  generator 
terminate#  will  not  drop  sufficiently  upon  the  application  of 
load.  The  less  saturated  a generator  is,  the  more  will  its  vol- 
tage decrease,  in  percent,  upon  the  application  of  a given  load. 


35 


The  increase  of  size  results  in  an  increase  of  cost  and  decrease 
of  portability. 

The  volt-ampere  curves  of  such  a generator  are  shaped 
practically  the  same  as  the  curve  for  the  self  excited  shunt  wound 
generator  shown  in  Figure  #3.  Such  curves  arc  not  very  desirable, 
for,  due  to  the  shape  of  the  lower  part  of  the  curve,  usually  not 
enough  short  circuit  current  v/ill  flow  to  quickly  heat  a spot  on 
the  work  to  the  melting  point.  Further  curves  are  not  drawn  out 
for  this  generator  because  the  machine  is  not  desirable,  on  ac- 
count of  its  sluggishness,  the  high  inductance  required,  its  size, 
and  its  volt- ampere  curves.  It  has  never  been  built  commercially 
for  arc  welding  service. 

21.  Self  Excited  Mdative  Compound  Wound  Generators.  - 
Two  types  of  self  excited  addative  compound  wound  generators  are 
actually  in  use.  In  the  first  type  the  generator  is  saturated 
magnetically  at  no  load,  sufficient  compounding  is  used  to  keep 
the  voltage  constant  regardless  of  load,  and  an  external  resis- 
tance is  used  to  reduce  the  voltage  at  the  arc  to  twenty  volts. 

In  the  second  type  the  generator  is  comparatively  unsaturated  at  no 
load  and  has  enough  series  to  produce  the  desired  welding  current, 
no  external  resistance  being  used. 

Two  varieties  of  the  first  type  are  used.  In  one  vari- 
ety an  adjustable  resistance  is  used  in  series  with  the  arc.  A 
diagram  of  the  generator  is  shown  in  Figure  13.  For  simplicity, 
and  to  show  the  difference  between  the  different  types  of  gener- 
ators, all  diagrams  -are  made  as  schematic  as  possible*  and  such 
things  as  arc  used  on  all  generators,  as  commutating  coils,  meters, 
line  switches  and  reactors,  are  omitted  from  the  figure.  Its  volt- 


3G 


Field  Rheostat 


Adjustable 
Re si  stance 


Fig.  6 - Diagram  of  a self  excited  addative  compound  wound 
saturated  generator  using  an  adjustable  external 
resistance. 


ampere  curves  is  the  same  as  those  shown  in  Figure  #5.  inis  gen- 
erator is  the  standard  size,  the  cheapest  and  the  most  portable 
of  arc  welding  generators.  It  however  has  an  efficiency  of  only 
about  36p,  when  resistance  losses  are  included,  and  the  cast  grid 
resistances  are  not  very  portable.  Such  a generator  is  simple, 
its  flux  changes  but  little,  so  speed  of  flux  action  is  not  impor- 
tant, and  only  enough  inductance  is  required  to  neep  the  electrode 
from  sticking  to  the  work,  when  striking  the  arc.  Its  volt-am- 
pere curve  is  satisfactory.  The  auxiliary  apparatus  required  is 
a voltmeter,  an  ammeter,  a rheostat,  adjustable  resistances,  with 
controlling  knife  switches,  and  a reactor.  This  type  of  genera- 
tor is  now  used  by  most  manufacturers  when  several  operators  are 
to  use  the  same  generator  and  formerly  was  manufactured  for  use 
as  a sin  ;le  operator  generator . Due  to  its  poor  efficiency,  it 


. 


3? , 


has  recently  been  largely  replaced  with  other  types  of  apparatus. 

The  other  generator  of  the  first  type  uses  a varying  re- 
sistance in  series  with  the  arc  instead  of  a permanent  adjustable 
one  • 

This  varying  resistance  consists  of  a carbon  pile  and  is 
varied  by  means  of  special  control  in  such  a way  that  the  current 
remains  approximately  constant.  Such  an  outfit  presents  a problem 
in  control  design  rather  than  generator  design  and  so  is  of  little 
importance  in  the  present  discussion.  So  far  as  generator  charac- 
teristics are  concerned,  excepting  only  the  volt-ampere  curve, 
this  generator  is  exactly  the  same  as  the  first  variation  con- 
sidered. This  type  of  apparatus  is  manufactured  by  the  Wilson 
Welder  and  Metals  Company,  Inc.,  under  the  name  of  the  Wilson 
Plastic  Arc  Welder.  As  a matter  of  interest,  it  may  be  stated 
that  the  Wilson  Welder  is  designed  to  produce  only  35  instead  of 
55  volts.  This  results  in  a much  smaller  generator  than  standard 
and  also  gives  a much  better  efficiency,  about  50>.  However,  due 
to  its  low  voltage,  such  generator  cannot  be  used  for  carbon 
electrode  welding,,  for  metallic  electrode  welding  where  the  elec- 
trode has  a heavy  coating,  nor  where  excessively  long  leads,  of 
a high  resistance  reach  from  the  welding  panel  to  the  work. 

The  second  type  of  self  excited  addative  compound  wound 
generator  is  not  designed  to  give  a definite  voltage  at  no  load. 
The  welding  current  is  adjusted  by  adjusting  the  strength. of  the 
shunt  field  by  means  of  a rheostat,  and  by  changing  the  strength 
of  the  series  field.  The  no  load  voltage  is  determined  by  the 
shunt  field  alone.  The  short  circuit  current  is  determined  by 
the  series  field  alone,  My  the  adjustment  of  both  fields  a volt- 


. 


* 


* 


38. 


ampere  curve  can  be  produced  through  almost  any  two  points  de- 
sired. The  rest  of  the  curve,  however,  cannot  be  controlled. 

In  actual  practice  the  big  changes  of  welding  current 
would  be  obtained  by  a change  of  series  field  strength  and  the 
finer  adjustments  made  by  varying  the  shunt  field  strength.  The 
volt-ampere  curves  of  this  generator  are  shown  in  Figures  #7  and 
#6.  Figure  #7  shows  the  effect  of  varying  the  series  field  alone. 
Figure  #8  shows  the  effect  of  varying  the  shunt  field  alone. 

These  curves  are  satisfactory  as  long  as  the  no  load  voltage  is 
not  too  low.  The  curves  with  an  extremely  low  no  load  voltage 
are  not  very  desirable. 

This  type  of  generator  has  very  wide  changes  of  flux. 

Its  fields  consist  of  a shunt  winding,  which  is  extremely  slug- 
gish, and  an  addative  series  winding,  which  was  found  to  have 
characteristics  that  tended  to  reduce  the  speed  of  flux  change. 

As  a result,  such  a generator  probably  would  hc-ve  a very  slow 
rate  of  flux  change,  with  a resulting  necessity  for  the  use  of  a 
highly  inductive  reactor  in  series  with  the  arc. 

The  efficiency  of  such  a generator  is  high,  and  it  is 
simple  in  construction.  Due,  however,  to  the  fact  that  it  can- 
not be  highly  saturated,  and  produce  the  volt-ampere  curves  de- 
sired, such  a generator  would  be  slightly  larger,  probably  about 
20/-,  than  the  standard  generator,  and  its  cost  would  increase  and 
portability  decrease  correspondingly. 

The  auxiliary  apparatus  required  is  a knife  switch,  a 
voltmeters,  an  ammeter,  a reactor,  a rheostat,  and  a means  for 
adjustin  the  strength  of  the  series  field.  Taps  could  be  brought 
out  from  the  series  field  and  means  provided,  eithei  xnife  switches 


\ 


41 


ox  dial  switches, for  connecting  to  the  desired  tap#  or  adjustable 
shunts  may  be  provided  for  the  series  field.  If  it  could  be 
easily  done  the  first  method  would  be  very  satisfactory.  However, 
to  bring  out  taps  it  is  necessary  to  bring  many  leads  out  of  each 
series  coil,  make  many  connections  inside  the  frame,  and  bring  sev- 
eral leads  out  from  the  generator  up  to  the  dial  switch  on  the 
panel.  All  of  these  leads  are  of  large  enough  cable  to  carry  the 
welding  current.  This  is  a very  awkward  and  costly  construction, 
i’he  last  method,  however,  is  not  to  be  recommended.  The  resistance 
of  the  series  field  is  low.  The  contact  resistance  of  the  knife 
switch,  or  of  the  dial  switch,  used  in  adjusting  the  series  shunt, 
is  subject  to  wide  variations,  and  may  exceed  the  resistance  of 
the  series  field  itself.  As  a result,  it  is  impossible  to  fore- 
tell definitely  the  percentage  current  shunted  from  the  series 
field,  and  consequently  the  welding  obtained  by  a given  setting 
is  indefinite. 

Until  very  recently,  this  type  of  arc  welding  gen- 
erator was  used  by  the  U.S.  Light  and  Heat  Corporation,  but  it  has 
now  been  abandoned  by  them.  A diagram  of  their  apparatus  is  shown 
in  Figure  #9.  This  type  is  not  now  being  built  commercially. 

22.  Separately  Excited  Differentially  Compound 
Wound  Generators.  - In  the  separately  excited  differential  compound 
generator  the  separately  excited  field  is  designed  to  produce  the 
desired  no  load  voltage  and  the  differential  series  field  is  de- 
signed to  reduce  the  voltage  from  the  no  load  voltage  to  the  ’weld- 
ing voltage  without  the  use  of  external  resistance. 

Generators  of  this  type  may  be  made  as  small  as  any 
welding  generator,  their  cost  should  be  very  low,  and  they  should 


43. 

be  readily  portable.  They  are  simple  and  have  a high  electrical 
efficiency. 

Such  generators  have  a wide  variation  of  main  pole  flux, 
but  their  speed  of  flux  change  is  unexcelled.  Therefore  they  re- 
quire but  little  inductance. 

Two  methods  are  used  in  adjusting  to  produce  the  de- 
sired welding  current.  The  usual  method  is  to  obtain  approximate- 
ly the  welding  current  desired  by  varying  the  strength  of  the 
series  field,  either  by  shunts,  cr  taps,  and  maxing  the  finer  ad- 
justments with  the  separately  excited  field  rheostat.  Generators 
of  this  type  are  manufactured  in  this  country  by  the  Lincoln  Elec- 
tric Company,  and  by  the  C.  and  C.  Electric  and  Manufacturing  Com- 
pany, and  are  manufactured  in  England  by  the  Metropolitan-Viewers 
Company.  A diagram  of  the  Lincoln  set  is  shown  in  Figure  #10.  An 
of  these  sets  use  an  adjustable  shunt  on  the  series  field,  which, 
as  stated,  is  objectionable.  The  volt-ampere  curves  to  be  ex- 
pected are  shown  in  Figure  #11,  and  are  very  desirable.  The  aux- 
iliary apparatus  required  is  a voltmeter,  an  ammeter,  a knife 
switch,  a reactor,  a rheostat,  some  means  for  changing  the  series 
field  strength,  and  a separate  source  of  excitation. 

The  other  method  of  adjusting  to  get  the  desired  weld- 
ing current  is  to  adjust  the  separately  excited  shunt  field  only, 
not  changing  the  series  field  strength  at  any  time.  This  scheme 
is  far  simpler,  and  gives  approximately  the  volt-ampere  curves 
shown  in  Figure  #12.  It  does,  however,  require  considerable  field 
space  for  the  shunt  coil,  for  a large  shunt  coil  must  be  used.  To 
illustrate,  assume  that  to  obtain  35  volts,  no  load,  (which  wi.l 
be  taken  as  a minimum),  1000  ampere  turns  are  required  per  pole. 


i 


I 


46 


and  1800  axe  required  to  obtain  55  volts.  Assume  that  to  obtain 
75  amperes  welding  current  the  difference  between  the  series  am- 
pere turns  per  pole,  and  the  separately  excited  shunt  ampere  turns 
per  pole,  will  be  700  ampere  turns,  and  that,  when  the  current  is 
to  be  175  amperes,  1625  ampere  turns  per  pole  are  required.  Then 

when  the  first  method  is  used  about  7 0r  15  series  turns 

75 

are  required, but  at  175  amperes  welding  the  series  field  is  re- 
duced to  the  strength  of  one  turn  and  the  separately  excited  field 
ampere  turns  is  1800  per  pole.  If  the  second  method  is  used  the 
series  turns  required  to  obtain  75  amperes  with  no  less  than  35 

volts  no  load  is  or  4 turns.  This  is  a minimum.  When 

75 

four  turns  are  used  for  175  ampere  welding  the  differential  series 
ampere  turns  becomes  175  x 4 or  700  ampere  turns,  and  the  total 
separately  excited  shunt  ampere  turns  become  700  + 1625  or  2325 
per  pole.  This  requires  more  field  copper,  and  more  field  space 
than  does  the  first  method.  The  shunt  field  would  have  a greater 
damping  action  at  high  current  than  would  the  field  for  the  first 
method,  due  to  the  greater  cross  section  of  copper.  At  low  weld- 
ing currents,  however,  due  to  the  higher  external  resistance  in 
the  shunt  field,  it  would  have  a smaller  damping  action.  Consider- 
ing the  effect  of  the  series  field  on  the  speed  of  action  the 
first  method  is  seen  to  have  an  advantage  at  low  currents,  due  to 
its  higher  number  of  effective  series  turns,  and  to  be  at  a dis- 
advantage at  high  currents,  due  to  its  then  lower  effective  number 
of  series  turns.  Balancing  the  damping  action  against  the  series 
effect  it  seems  there  is  little  difference  between  the  speed  of 
action  of  the  two  machines. 

The  diagram  for  this  generator  is  the  same  as  that  of 


47. 

Figure  #10  except  that  there  is  no  adjustable  shunt  on  the  series 
fields.  The  auxiliary  apparatus  required  is  a voltmeter,  an  am- 
meter, a knife  switch,  a separate  source  of  excitation,  a rheo- 
stat and  a reactor.  This  type  of  generator  has  not  been  manufac- 
tured commercially. 

23.  Separately  Excited  Self  Excited  Shunt  Wound  Gen- 
erators. - A generator  with  a separately  excited  shunt  field  and 
a self  excited  shunt  field  can  be  adjusted  to  give  an  ideal  arc 
welding  volt-ampere  curve.  To  adjust  the  welding  current  either 
one  or  two  adjustments  may  be  used.  A rheostat  should  be  connect- 
ed in  series  with  the  separately  excited  field,  for  welding  ad- 
justment, and  a rheostat  may  be  connected  in  series  with  the  self 
excited  shunt  field.  If  only  one  rheostat  is  used,  the  type  of 
volt-ampere  curves  shown  in  Figure  #13  is  obtained  and  if  two 
rheostats  are  used  almost  any  shape  of  curves  may  be  produced,  de- 
pending on  the  relative  adjustments.  However,  the  best  type  of 
curves  tc  be  obtained  are  shown  in  Figure  #14.  Either  set  of 
curves  are  very  desirable. 

Considering  only  its  speed  of  action,  the  separately 
excited  shunt  wound  generator  is  not  well  suited  for  welding  ser- 
vice. The  flux  has  a wide  range  of  change,  and  the  dependence  on 
a self  excited  shunt  field  to  cause  this  change  results  in  a 
’"slow”  generator.  The  larger  the  percentage  that  the  self  excited 
shunt  ampere  turns  is  of  the  total  ampere  turns,  the  slower  will 
be  the  flux  rate  of  change.  A self  excited  shunt  field  is,  as 
previously  stated,  very  slow  in  action.  Due  to  this  sluggishness 
the  self  excited  separately  excited  generator  requires  the  use  of 
a large  amount  of  inductance  if  used  for  welding  service. 


50 


This  type  of  generator  can  be  made  as  small  as  any  other 
generator  for  the  service.  It  is  very  simple  in  construction, 
but  if  two  rheostats  are  used  the  operating  adjustments  may  be- 
come complex  and  the  wrong  shape  of  volt-ampere  curve  is  as  like- 
ly to  be  produced  as  is  the  right  one.  The  cost  of  such  a gener- 
ator is  low,  and  it  is  readily  portable  by  truck. 

The  auxiliary  apparatus  required  for  this  generator  is  a 
voltmeter,  an  ammeter,  a knife  switch,  one  or  two  rheostats,  a re- 
actor and  a separate  source  of  excitation.  A typical  schematic 
diagram  of  such  a generator,  using  one  rheostat,  is  shown  in  Fig- 
ure #15.  A separately  excited  self  excited  shunt  wound  genera- 
tor is  now  being  manufactured  commercially  by  the  U.S.  Light  and 
Heat  Corporation,  and  replaces  the  self  excited  addative  series 
generator  which  they  formerly  built. 

24.  Separately  Excited  Self  Excited  Differential  Com- 
pound Generators.  - The  separately  excited  self  excited  differen- 
tial compound  wound  generator  is  at  present  the  most  favored  of  all 
single  operator  welding  generators  by  manufacturers  of  such  ma- 
chinery. Such  generators  may  have  their  welding  current  con- 
trolled by  almost  any  combination  of  self  excited  shunt  field  ad- 
justment, separately  excited  shunt  field  adjustment,  and  series 
field  adjustment.  This  type  of  generator  is  a compromise  between 
the  separately  excited  differential  compound  wound  generator,  and 
the  separately  excited  self  excited  generator.  In  size  all  three 
are  the  minimum  possible,  and  all  three  are  readily  portable  by 
truck.  The  generator  'with  three  separate  fields,  however,  is  less 
simple,  and  consequently,  slightly  more  costly* 

Its  speed  of  flux  change  is  better  than  that  of  the  self 


Field  Rheostat 

/ \ 


51 , 


Separate 

Excitation 


Arc 


Fig.  15  - Diagram  of  a separately  excited,  self  excited  shunt 
wound  generator,  with  one  field  adjustment. 


Field  Rheostat 


Adjustable  Differential 


Fig.  16  - Diagram  of  a separately  excited,  self  excited  differ- 
ential compound  wound  generator,  with  all  fields 
adjustable . 


52. 

excited  separately  excited  generator,  but  is  not  so  good  as  that 
of  the  separately  excited  differential  compound  generator.  The 
relative  strength  of  the  three  fields  determines  'which  it  most 
nearly  approaches.  The  weaker  the  self  excited  field,  and  the 
stronger  the  separately  excited  and  the  differential  series  fields 
the  faster  is  the  rate  of  flux  change.  The  inductance  required 
is  dependent  on  the  speed  of  flux  change. 

The  auxiliary  apparatus  required  by  this  generator  is  a 
voltmeter,  an  ammeter,  a knife  switch,  one  or  two  rheostats,  a 
source  of  separate  excitation,  possibly  some  means  of  adjusting 
the  series  field,  and  a reactor.  A diagram  showing  this  generator 
and  the  maximum  of  auxiliary  apparatus  (except  meters,  reactor, 
and  knife  switch)  is  shown  in  Figure  #16. 

When  adjustment  is  made  only  of  the  separately  excited 
field  the  volt-ampere  curves  would  be  about  as  shown  in  Figure  #17. 
These  are  desirable  curves.  The  curves  at  the  low  welding  currents 
are  less  desirable  than  those  of  the  high  welding  currents. 

When  adjustment  is  made  of  the  self  excited  field  only, 
the  volt-ampere  curves  produced  are  approximately  as  shown  in  Fig- 
ure #18.  These  curves  are  acceptable,  although  the  tendency  is 
for  the  no  load  voltage  to  be  too  low  when  the  rheostat  is  set  for 
low  welding  currents,  and  for  the  short  circuit  current  to  be  too 
low  when  the  rheostat  is  set  for  high  welding  currents. 

When  rheostats  are  put  in  series  with  both  the  separate- 
ly excited  field  and  the  self  excited  field,  many  different  shapes 
of  volt  ampere  curves  may  be  obtained.  When  adjusting  for  a given 
welding  current  the  double  adjustment  permits  the  operator  to  ob- 
tain either  good  or  bad  volt-ampere  curves.  At  their  best  the 


: 


55. 


curves  appear  about  as  shown  in  Figure  #14  for  the  self  excited 
separately  excited  generator.  Such  double  adjustments  maxe  the 
operation  of  the  generator  complicated. 

The  strength  of  the  series  field  might  also  be  adjusted 
in  addition  to  the  adjustment  of  one  or  both  of  the  other  fields. 
In  actual  practice,  this  has  not  been  done  commercially  for  the 
reason  that  it  complicates  the  control,  and  is  not  necessary  to 
obtain  satisfactory  volt-ampere  curves.  In  Figure  #18  the  dotted 
curves  indicate  the  volt-ampere  curves  that  would  be  obtained  if 
the  differential  series  field  were  strengthened. 

The  American  companies  manufacturing  plain  separately 
excited,  self  excited,  differential  series  generators  both  use 
rheostats  in  series  ’with  both  the  self  excited  shunt  and  the  sep- 
arately excited  fields.  These  companies  are  the  Siemund  Wenzel 
Electric  Welding  Company,  and  the  Burke  Electric  Company.  English 
manufacturers  making  separately  excited  self  excited  differential 
series  welding  generators  are  Metropolitan-Viewers  Company,  Ltd., 
(Formerly  the  British  West inghouse  Electric  and  Manufacturing  Com- 
pany, Ltd.),  the  Premier  Electric  Welding  Company,  Crompton  and 
Company,  Ltd.,  and  the  Lancashire  Dynamo  and  Motor  Company,  Ltd. 

Of  these  all  but  the  last  one  use  rheostats  in  series  with  both 
the  self  excited  shunt  field  and  the  separately  excited  field. 

The  last  named  however  provides  for  adjustment  of  the  self  excited 
shunt  field  only. 

25.  Interconnected  Generators.  - When  generators  re- 
quiring separate  excitation  are  used,  it  is  possible  to  intercon- 
nect the  exciter  circuit  and  the  welding  circuit  in  such  a way 
that  the  exciter  voltage  is  impressed,  through  a resistance,  on 


. . 


* 


I 


I 


t 


I 


53 . 


the  arc.  Figures  #19,  #20,  and  #21  show  diagrams  of  simple  inter- 
connected generators.  These  generators  are,  then,  consequently  a 
compromise  between  the  generator  before  interconnection,  and  a 
plain  resistance  'welder,  discussed  in  Section  19.  Since  the  cur- 
rent taken  from  the  exciter  circuit  is  usually  small,  the  volt- 
ampere  curves  are  not  materially  affected  by  interconnection. 

Since  the  resistance  welder  has  an  instantaneous  voltage  response 
to  changes  of  current,  the  interconnection  would  make  the  arc  a 
little  more  stable  and  tenacious  than  it  would  be  without  the  in- 
terconnection. Such  interconnection  does  not  affect  the  genera- 
tor size.  It  does  however  require  a larger  exciter  than  would 
otherwise  be  needed;  requires  that  an  extra  resistance  be  used  in 
the  auxiliary  apparatus;  decreases  the  electrical  efficiency,  due 
to  the  loss  in  the  resistance;  and  makes  the  set  as  a whole  more 
complicated,  more  expensive  to  build,  and  harder  to  repair,  hone 
of  these  three  types  are  being  built  commercially. 

A special  type  of  interconnected  generator  is  shown  in 
the  diagram  of  Figure  #22.  This  is  essentially  a separately  ex- 
cited, self  excited,  differential  compound,  interconnected  gener- 
ator, and  its  volt-ampere  curves  would  be  similar  to  those  shown 
in  Figure  #17.  Due  to  its  very  peculiar  connections  an  analysis 
of  this  generator  is  of  interest.  If  resistances  Hi  and  R-  are 
taken  as  shown  in  Figure  #22,  the  resistance  of  the  self  excited 
field,  sometimes  called  the  reversing  field,  is  R3,  the  exciter 
voltage  is  E,  and  is  constant,  while  the  welding  generator  voltage 
is  ^2,  and  currents  Iq,  I 3 and  I 3 are  assumed  as  shown  in  the  dia- 


gram, then: 


59. 

Ei  = IX  Ri  + IS  H2 
E2  = I2  R2  + I3  R3 
I1  + I3  - I2 

E1  * 1Z  R1  " I3  R1  + I2  R2 
E2  * I2  R2  + *3  R3 

El  = (Rx  + Ra)  I2  - I3  Hi 
E2  = I2  r2  + *3  R3 

R2  E1  = R3  (Rx  + Rs  ) I2  - I3  Rx  R2 

(Rl  + R2  ) E2  *=  R2  (r1  + r2^  ^2  + (R1  + r2  ) R3  ^3 

(R1  + r2  ) E2  - R2  E1  - h R1  R2  + (R1  + R2  > R3  h 

I _ (Rl  + R2  ) S2  - Rs  Ei 

3 Rx  R2  + (R1  + Rs  ) R3 

R1  R2  + R1  R3  + R2  R3 

All  of  the  resistances  are  constants.  Then  let 

R-,  + R0  r 

_ — _ — — _ = Ct  and  J2 

R1  n2  + R1  R3  + r2  k3  Ri  R2  + Ri  R3  + R2  R3~  w2 

Then  1 3 = G1  E2  — C2 

If  the  effective  turns  per  pole  of  the  self  excited 
field  is  Ti,  then  the  self  excited  field  ampere  turns  per  pole  be- 
comes C-j_  E2  Tx  - C 2 Ei  Ti«  Assume  the  separately  excited  field 
has  an  ampere  turns  per  pole  of  C3  Ei  Ti.  Then  the  total  ampere 
turns  per  pole  due  to  the  two  fields  is  equal  to  Ci  E2  - 


6Q« 


Cg  E-j_  T^  + C3  T^_  = (C3  + Cg  ) T-j_  + C^  Eg  T^.  Exactly  the 
same  ampere  turns  per  pole  could  have  been  obtained  by  using  a 
separately  excited  field  of  the  constant  ampere  turns  (C3  - Cg) 

Ei  Ti  and  a self  excited  shunt  field  with  Eg  ampere  turns 
per  pole,  and  using  connections  as  shown  in  Figure  #21. 

As  compared  to  the  generator  whose  diagram  of  connec- 
tions is  shown  in  Figure  #21,  the  volt-ampere  curves  are  the  same. 
The  speed  of  action  of  the  generator  of  Figure  #22  is  probably 

I 

superior,  for  it  has  a comparatively  high  resistance  in  series 
with  the  self  excited  shunt  coil,  which  tends  to  reduce  the  damp- 
ing action  of  that  coil.  Consequently  less  inductance  would  be 
required.  This  resistance,  however,  causes  an  extra  loss  that 
lowers  the  generator  efficiency  slightly,  also  this  extra  resis- 
tance Rg  increases  the  cost  of  the  apparatus  slightly  and  makes  it 
slightly  more  difficult  to  assemble  and  repair.  The  size  and  port- 
ability i3  not  affected.  This  type  of  generator  is  at  present 
manufactured  by  the  West ingho use  Electric  and  Manufacturing  Com- 
pany. 

A very  similar  type  of  interconnected  generator  is 
shown  in  the  diagram  in  Figure  #23.  It  will  produce  exactly  the 
same  volt-ampere  curves  if  proper  adjustments  are  mads  as  will  the 
generator  last  discussed. 

Using  the  terms  and  Rg  for  the  resistances,  with  a 
given  setting  of  the  three  point  rheostat,  as  shown  in  Figure  #23, 
R3  for  the  combined  resistance  of  the  shunt  field  and  its  rheostat, 
as  used,  If,  Ig,  and  I3  for  the  currents  through  these  resistances, 
El  for  the  constant  exciter  voltage  and  Eg  for  the  welding  gener- 
ator voltage,  then 


. 


. 


* 


. 


. 


62, 


E1  = I1  Ri  + X3  R3 
E2  = I3  R3  - I2  R2 

"Is  *2  + *3 


E1  ~ I2  R1  + X3  R1  + I3  R3 
e2  * I3  R3  + I2  R2 


Ei  R2  s*  Ri  R2  I g + (Ri  R2  + R2  R3)  1 3 

E2  Ri  = R!  r2  I2  + Ri  R3  Is 

E1  R2  + e2  R1  = (R1  r2  + r2  R3  + R1  r3^  *3 

I3  * E1  r2  + e2  r1 

R1  R2  + R1  r3  + R2  R3 

If  in  this  case  T'i  is  the  number  of  effective  turns 
per  pole  of  the  shunt  field,  (C3  - C2)  be  allowed  to  equal 


p,_,  and  Ci  to  be 


R1 


, then 


Rl  R2  + Rp  R3  + R2  R3'  Ri  R2  + Rl  R3  + R2  r3j 

the  effective  shunt  ampere  turns  per  pole  becomes  (C3  - C2)  hp  fi'i 

+ Ci  e2  t1. 

This  is  exactly  the  same  formula  as  was  obtained  for 
the  last  interconnected  generator  discussed.  It  indicates  that  if 
the  fields  are  properly  designed,  and  the  adjustments  properly 
made,  the  total  ampere  turns  per  pole  due  to  shunt  fields,  sepa- 
rately excited  and  self  excited,  would  be  exactly  the  same  on 
either  machine  when  both  have  the  same  generator  voltage.  With 
proper  adjustments,  then,  the  same  volt-ampere  curves  should  be 
obtained.  The  three  point  rheostat  is  adjusted  primarily  to  ob- 


tain the  saturation  curve  shape  desired.  It  changes  the  relative 


63. 


strength  of  the  separately  excited,  and  of  the  self  excited,  ac- 
tion of  the  shunt  field.  This  rheostat  could  be  eliminated  and. 

Hp  and  Rg  left  as  constant,  instead  of  adjustable,  resistances. 

This  would  give  the  operator  less  control,  and  would  probably  in- 
sure the  use  of  better  volt-ampere  curves  than  would  be  otherwise 
used# 

The  field  rheostat  is  adjusted  primarily  to  control  the 
magnitude  of  the  welding  current. 

This  type  of  interconnected  generator  gives  the  same 
volt-ampere  curves  as  does  the  interconnected  generator  last  des- 
cribed; it  has  the  same  speed  of  action,  the  same  efficiency,  the 
same  inductance  and  is  the  same  size.  However  this  generator  it- 
self is  slightly  more  simple  in  construction,  as  it  has  only  one 
set  of  shunt  field  coils  instead  of  two.  Therefore  the  generator 
would  be  slightly  cheaper,  and  easier  to  repair.  This  typ9  of 
generator  is  not  at  present  being  manufactured  commercially, 

26,  Self  Excited,  Differential  Series,  Third  Brush 
Generator.  - A review  of  the  types  of  single  operator  welding  gen- 
erators discussed  will  show  that  every  generator  that  was  entire- 
ly self  excited  either  had  a very  low  efficiency  or  a very  slow 
speed  of  flux  change.  All  the  generators  considered  were  ordinary 
generators  except  for  specially  designed  field  windings.  An  ex- 
citer necessarily  adds  complication  to  a set  and  maxes  it  more  ex- 
pensive. Many  schemes  have  been  suggested  for  an  entirely  self 
excited  welding  generator  that  would  have  no  undesirable  welding 
characteristics,  but  every  such  scheme  required  a very  special  type 
of  generator  construction. 

One  of  the  generators  suggested  is  the  self  excited 


. . ! 


. 


* 


64. 


differential  series  third  "brush  generator  whose  diagram  is  shown 
in  Figure  #24.  The  construction  of  this  generator  is  much  the 
same  as  used  for  third  brush  automobile  battery  charging  genera- 
tors. However  a differential  series  field  is  added  and  the  shunt 
field  is  connected  between  brushes  B and  C instead  of  A and  B. 

If  a generator  is  saturated  magnetically  at  no  load 
both  the  pole  tips  are  then  usually  saturated.  If  load  is  thrown 
on  the  generator  the  armature  ampere  turns  tend  to  saturate  fur- 
ther the  trailing  pole  tips.  Even  with  no  field  excitation,  the 
trailing  pole  tip  will  be  saturated  if  a very  large  current  is 
flowing.  In  Figure  #35  is  shown  the  voltage  that  would  be  induced 
in  a conductor  of  the  rotating  armature  at  no  load  and  at  short 
circuit.  It  i3  assumed  that  there  is  a slight  excitation  at  short 
circuit  and  that  therefore  some  current  is  flowing.  The  brushes 
A,  B,  and  C shown  indicate  the  setting  of  the  brushes.  The  total 
induced  voltage  from  B to  C is  proportioned  to  the  area  shown 
under  the  voltage  curve  between  these  two  points.  It  is  evident, 
then,  that  if  the  voltage  drops  due  to  load,  the  trailing  pole  tip 
remains  saturated,  and  the  voltage  between  brush  B and  brush  C 
remains  approximately  constant.  This  voltage  can  be  used  to  re- 
place the  separate  exciter,  and  volt-ampere  curves  approximately 
as  shown  in  Figure  #12  would  be  obtained. 

If  the  brush  B is  shifted  toward  brush  A it  will  be 
noted  that  voltage  B-C  will  increase  at  no  load,  but  that  it  will 
decrease,  instead  of  remaining  constant,  when  the  load  comes  on, 
and  that  the  amount  of  this  decrease  depends  on  the  position  of 
the  brush  B.  It  can  be  set  to  get  any  percentage  decrease  that  is 
desired  from  approximately  none  to  approximately  100$  decrease. 


66 


Then  the  setting  of  brush  B will  determine  not  only  the  excita- 
tion voltage  but  also  the  degree  in  which  the  generator  will  act 
like  a separately  excited,  or  like  a self  excited,  generator. 

It  should  be  noted  that,  in  order  to  obtain  a high 
inductance,  the  pole  tip  should  never  be  saturated.  Therefore 
actually  the  peak  of  the  voltage  curve  will  probably  be  higher  at 
short  circuit  with  full  load  current,  than  it  will  be  at  no  load. 
This  can  be  compensated  for  by  the  setting  of  brush  B. 

The  welding  current  is  controlled  by  means  of  a rheo- 
stat in  the  shunt  field.  The  brush  B should  be  set  in  position 
and  not  moved. 

Such  a generator  has  the  size,  portability,  efficiency, 
volt-ampere  curves,  and  speed  of  flux  action  of  the  corresponding 
separately  excited  generator,  not  interconnected.  It  requires  the 
same  auxiliary  equipment  as  does  the  separately  excited  differen- 
tial series  generator,  except  that  the  exciter  is  omitted.  Its 
field  windings  are  as  simple  as  those  of  the  separately  excited 
differential  series  generator.  It  requires,  however,  a rather  com- 
plicated brush  rigging,  and  the  brush  B at  times  short  circuits 
bars  between  'Which  there  is  a difference  of  potential  of  approxi- 
mately three  volts.  This  tends  to  cause  sparking  under  brush  3, 
but  this  tendency  can  be  made  small  by  using  high  resistance,  and 
very  narrow,  brushes.  As  brush  B takes  only  excitation  current 
from  the  armature  there  is,  practically,  no  reactance  voltage  to 
overcome,  and  a very  narrow  brush  can  be  used.  This  type  of  gen- 
erator is  not  being  manufactured  commercially. 

27,  Self  Excited  Differential  Series  Split  Pole  Gen- 
erator. - One  of  the  schemes  for  obtaining  a single  operator 


67. 

welding  generator  which  not  only  has  desirable  welding  character- 
istics, but  which  also  does  not  require  an  exciter,  is  the  use  of 
a self  excited  differential  series  split  pole  generator.  Such  a 
generator  is  shown  in  Figure  #26.  The  generator  has  a four  pole 
stator,  and  a two  pole  rotor.  The  welding  current  is  taken  from 
brushes  A and  C,  and  the  excitation  current  from  A and  B.  Poles 
2 and  4 with  brushes  A and  E constitute  a two  pole  constant  po- 
tential shunt  wound  generator.  The  position  of  brushes  A and  C 
is  such  that  the  armature  reaction,  due  to  the  welding  current, 
will  tend  to  increase  the  flux  in  poles  3 and  4.  This  however 
can  be  done  in  only  a very  slight  degree,  as  these  poles  are 
notched  and  kept  saturated.  The  poles  are  proportioned  sc  that 
the  voltage  between  brushes  A and  E is  approximately  thirty  volts. 
The  poles  1 and  3 are  unsaturated.  They  have  a shunt  winding  that 
is,  in  effect,  separately  excited,  as  constant  excitation  is  fur- 
nished by  the  voltage  between  brushes  A and  B.  They  also  have  an 
adjustable  differential  series  winding.  Furthermore,  the  arma- 
ture reaction  due  to  the  welding  currents  is  such  as  to  tend  to 
reduce  the  flux  in  poles  1 and  3,  and  so  the  armature  itself  acts 
on  the  flux  of  poles  1 and  3 in  the  same  way  as  would  a differen- 
tial series  field  with  a constant  number  of  turns.  The  shunt 
field  and  the  poles  are  designed  to  produce  30  volts  at  no  load. 
Than  at  no  load  the  voltage  between  brushes  A and  B is  30  volts, 
between  B and  C is  30  volts,  and  between  A and  C is  50  volts.  As 
load  comes  on  the  armature  and  the  differential  series  ampere 
turns  overbalance  the  shunt  ampere  turns,  till,  at  short  circuit, 
the  voltage  induced  by  poles  1 and  3 is  approximately  30  volts  in 
the  opposite  direction  from  what  it  was  at  no  load.#  The  voltage  be— 


Field  Rheostat 


68 


o 

u 

<3 


Diagram  of  a self  excited  differential  compound  split  pole 
generator  with  both  fields  adjustable. 


69. 


tween  brushes  A and  B then  will  be  30  volts,  the  voltage  between 
B and  C will  be  approximately  30  volts  in  the  opposite  direction. 
Then  at  short  circuit  the  net  volts  induced  between  brushes  A and 
C is  approximately  zero.  If  the  magnetic  circuit  through  poles 
1 and  3 is  unsaturated,  then  the  volt  ampere  curves  would  be  ap- 
proximately as  shown  in  Figure  #27,  and  appear  as  straight  lines. 
If,  however,  the  magnetic  circuit  through  poles  1 and  3 is  satur- 
ated at  no  load,  a curve  such  as  is  shown  in  Figure  #28  is  pro- 

duced. Either  curve  is  satisfactory  for  welding  purposes. 

The  control  of  the  welding  current  is  obtained  princi- 
pally by  adjusting  the  strength  of  the  series  field.  When  the 
approximate  welding  current  desired  is  obtained,  then  the  shunt 
field  strength  is  adjusted  to  give  the  current  desired. 

The  speed  of  flux  change  cf  such  a generator  is  very 

rapid.  Its  speed  cf  flux  change  is  the  same  as  is  that  of  a 

separately  excited  differential  series  generator.  Consequently 
little  inductance  is  required. 

This  type  of  generator  is  nearly  as  efficient  as  any 
other  type.  The  two  pole  armature  is  less  efficient  than  the 
four  pole  and  so  slightly  decreases  the  generators  efficiency. 

Such  a generator  will  be  larger,  and  consequently  more 
costly  and  less  portable,  than  the  standard  generator.  A two  pole 
armature  has  longer  end  connections  than  does  a four  pole  armature, 
and  so  has  a greater  loss  there.  Its  ventiliation  is  usually 
poorer,  and  so  it  can  dissipate  less  loss.  As  a result  of  these 
two  factors,  a two  pole  armature  is  larger  than  a four  pole  one 
of  the  3ame  rating.  Also,  if  part  of  the  magnetic  circuit  is  made 
unsaturated  at  no  load,  then  the  generator  size  must  increase  ac- 


72. 


cor&ingly.  Furthermore  the  welding  current  passes  between  the 
brushes  and  the  commutator  at  only  two  places  on  the  commutator 
instead  of  four.  Consequently,  for  the  same  current  rating  a 
two  pole  armature  will  have  a commutator  approximately  twice  as 
long  as  will  a four  pole  armature. 

The  cost  of  this  type  of  generator  would  be  high  on  ac- 
count of  its  size  and  its  special  construction. 

Such  a generator  is  not  very  simple  of  assembly,  for 
all  poles  are  not  the  same,  all  fields  are  not  the  same,  and 
brushes  are  not  symmetrically  placed. 

For  auxiliary  apparatus  is  required  a voltmeter,  an  am- 
meter, a knife  switch,  a field  rheostat,  a reactor,  and  means  of 
adjusting  the  series  field  strength.  This  type  of  generator  is 
manufactured  by  the  General  Electric  Company.  A3  a means  of  ad- 
justing the  series  field  strength  they  bring  out  taps  from  the 
series  coil  to  the  panel,  and  cut  turns  in  or  out  by  a series 
field  dial  switch,  instead  of  using  the  adjustable  shunt  shown 
in  Figure  #26. 


73. 


PART  V. 

CONCLUSION 

28.  Present  Stage  of  Development.  - Arc  welding,  in 
itself,  is  not  a new  art.  It  was  used  as  early  as  1881.  At  the 
time  of  the  outbreak  of  the  World  War,  arc  welding  was  used  to 
a very  limited  extent  in  railroad  shops  and  factories.  During 
the  war  it  was  largely  used  for  the  repair  of  old  ships  and  the 
building  of  new  ones.  During  the  war,  and  since,  the  process  has 
been  rapidly  growing  in  popularity,  for  it  definitely  proved  its 
value  in  the  shipyards.  Consequently,  most  of  the  development 
of  single  operator  arc  welding  generator  has  taken  place  in  the 
last  five  years,  although  a few  were  developed  before  then. 

Many  different  types  of  generators  were  produced  that 
would  weld  satisfactorily.  These  generators,  and  a few  other 
possible  types,  have  been  analyzed.  A summary  of  the  analysis 
is  shown  in  Figure  #29.  The  generators  corresponding  to  the 
various  numbers  are  as  follows: 

1.  Separately  excited  generator. 

2.  Self  excited  shunt  wound  saturated  generator. 

3.  Self  excited  shunt  wound  unsaturated  generator. 

4.  Self  excited  addative  compound  wound  saturated 

generator. 

5*  Self  excited  addative  compound  wound  unsaturated 
generator . 

6.  Separately  excited  differential  compound  wound 
adjustable  series  generator. 


74. 


7.  Separately  excited  differential  compound  'wound 

constant  series  generator. 

8.  Separately  excited  self  excited  shunt  wound  gen- 

erator with  one  field  adjustment* 

9.  Separately  excited  self  excited  shunt  wound  gen- 

erator with  two  field  adjustments. 

10.  Separately  excited  self  excited  differential  com- 

pound wound  generator,  separate  field  adjust- 
able . 

11.  Separately  excited  self  excited  differential  com- 

pound wound  generator,  self  excited  shunt 
field  adi us table. 

w 

12.  Separately  excited  self  excited  differential  com- 

pound wound  generator,  both  shunt  fields  ad- 
justable. 

13.  Simple  separately  excited  differential  compound 

wound  interconnected  venerator. 

14*  Simple  separately  excited  self  excited  shunt  wound 
interconnected  generator. 

15.  Simple  separately  excited  self  excited,  differen- 

tial compound  wound  interconnected  generator. 

16.  First  special  (Wastinghouse)  type  of  separately 

excited  self  excited  differential  compound 
wound  interconnected  generator 

17.  Second  special  type  of  separately  excited  self 

excited  differential  compound  wound  inter- 
connected generator. 


75. 


IS.  Self  excited  differential  compound  wound  third 
brush  generator. 

19.  Self  excited  differential  compound  wound  split 
pole  generator. 

these  generators  were  graded  with  respect  to  their  various  char- 
acteristics. A is  the  best  grade.  It  should  be  understood  that 
though  in  the  tabulation  one  type  is  shown  better  than  another 
in  regard  to  some  special  characteristic,  it  does  not  necessari- 
ly follow  that  a specific  generator  of  the  first  rype  will  be 
better  than  a specific  generator  of  the  second  type  in  regard  to 
that  characteristic.  This  would,  however,  be  the  case  if  the 
two  generators  were  equally  well  designed  in  all  other  respects. 
The  separately  excited,  self  excited,  differential  compound 
wound  generator  is  rated  as  having  a better  volt- ampere  curve 
than  the  separately  excited  differential  series  generator.  That 
does  not  mean  that  if  a generator  of  each  type  were  taken  the 
generator  of  the  first  type  would  necessarily  have  the  best  volt- 
ampere  curve.  It  does,  however,  mean  that  a generator  of  the 
first  type  can  be  designed  to  have  a better  volt-ampere  curve 
than  can  a generator  of  the  second  type. 

Also, it  should  be  understood  that  no  attempt  has  been 
made  to  analyze  every  possible  generator  that  could  be  used  for 
single  operator  arc  welding.  A few  of  the  simplest  generators 
not  in  use,  and  the  more  important  generators  in  use,  were  con- 
sidered. 

29.  Probable  Future  Development.  - Although  up  to  the 
present  has  been  a period  of  development,  the  future  appears  to 


76 


be  a period  of  keen  competition,  with  its  resultant  elimination 
of  types.  The  generators  that  have  the  least  desirable  welding 
characteristics  must  go.  The  costly  generators  must  go.  Any 
generators  that  are  unduly  large,  or  complicated  must  go. 

A comparison  of  the  xypes  as  shown  in  Figure  #28  would 
indicated  that  the  separately  excited  differential  generator  with 
no  adjustment  of  the  series  field  is  the  most  probable  survivor. 

It  is  evident  that  all  single  operator  generators,  whether  sepa- 
rately excited,  shunt  wound,  or  addative  compound  wound,  that  de- 
pend on  resistance  to  reduce  the  voltage  from  55  to  20  cannot  sur- 
vive, for  their  efficiency  is  too  low.  The  self  excited  addative 
compound  wound  unsaturated  generator  probably  could  not  compete, 
for  its  field  action  is  too  slow.  The  separately  excited  differ- 
ential compound  wound  generator  with  an  adjustable  series  will 
probably  disappear  due  to  the  difficulty  of  making  satisfactory 
provision  for  adjusting  the  series.  The  separately  excited  seif 
excited  shunt  generator  will  probably  fail  because  of  its  slow 
field.  The  self  excited  separately  excited  differential  compound 
wound  generator  is  probably  too  costly  to  compete.  The  same  thing 
is  true  of  all  interconnected  generators.  The  self  excited  dif- 
ferential compound  wound  third  brush  generator  might  fail  because 
of  sparking  under  the  third  brush.  The  self  excited  differential 
series  split  pole  generator  probably  could  not  compete  due  to  its 
high  cost  and  size.  It  is  interesting  to  speculate  upon  which 
types  will  survive,  but  only  time  will  tell  with  certainty. 


77 


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Pig.  29  - Single 
Operator  arc  welding 
generator  comparison. 


